appdata: Update appdata

- Mark release descriptions as untranslatable (1)
- Remove keywords that already presented in the desktop file (2)
- Add vcs-browser and translate URLs

1) GNOME automatically excludes release descriptions on Damned Lies (GNOME Translation Platform). It's a good practice to follow the GNOME way.

This can streamline the translation process, allowing translators to focus their efforts on more critical and user-facing aspects of the application.

2) More information: https://docs.flathub.org/docs/for-app-authors/appdata-guidelines/#icons-and-categories

data/com.mattjakeman.ExtensionManager.desktop.in.in

@@ -9,4 +9,4 @@ StartupNotify=true
 MimeType=x-scheme-handler/gnome-extensions;
 Comment=Manage GNOME Shell Extensions
 # Translators: Do not translate
-Keywords=extension;manager;
+Keywords=extension;manager;shell;

data/com.mattjakeman.ExtensionManager.metainfo.xml.in.in

@@ -7,7 +7,7 @@
   <summary>Browse, install, and manage GNOME Shell Extensions</summary>
   <content_rating type="oars-1.1" />
   <developer_name translatable="no">Matthew Jakeman</developer_name>
-	<description>
+	<description translatable="no">
 	  <p>A utility for browsing and installing GNOME Shell Extensions.</p>
     <p>Features:</p>
     <ul>
@@ -32,11 +32,9 @@
   <launchable type="desktop-id">@app_id@.desktop</launchable>
   <url type="homepage">https://github.com/mjakeman/extension-manager</url>
   <url type="bugtracker">https://github.com/mjakeman/extension-manager/issues</url>
+  <url type="vcs-browser">https://github.com/mjakeman/extension-manager</url>
+  <url type="translate">https://github.com/mjakeman/extension-manager/blob/master/po/README.md</url>
   <url type="help">https://github.com/mjakeman/extension-manager/discussions</url>
-  <keywords>
-    <keyword>Extensions</keyword>
-    <keyword>Shell</keyword>
-  </keywords>
   <translation type="gettext">extension-manager</translation>
   <supports>
     <control>pointing</control>
@@ -48,7 +46,7 @@
   </requires>
   <releases>
     <release version="0.4.3" date="2023-11-08">
-      <description>
+      <description translatable="no">
         <ul>
           <li>HOTFIX: Fix broken 'recent' and 'popularity' search filters</li>
           <li>Various stability and correctness fixes</li>
@@ -57,7 +55,7 @@
       </description>
     </release>
     <release version="0.4.2" date="2023-06-19">
-      <description>
+      <description translatable="no">
         <ul>
           <li>Translation updates</li>
           <li>Upgrade Assistant scroll fix</li>
@@ -66,7 +64,7 @@
       </description>
     </release>
     <release version="0.4.1" date="2023-05-06">
-      <description>
+      <description translatable="no">
         <ul>
           <li>Translation updates</li>
           <li>Official snap support</li>
@@ -75,7 +73,7 @@
       </description>
     </release>
 		<release version="0.4.0" date="2022-09-26">
-			<description>
+			<description translatable="no">
 				<ul>
 					<li>Fully adaptive mobile-friendly user interface</li>
 					<li>Paginated search results</li>
@@ -90,7 +88,7 @@
 			</description>
 		</release>
     <release version="0.3.2" date="2022-08-17">
-      <description>
+      <description translatable="no">
         <ul>
           <li>Translation updates</li>
           <li>Various bug fixes</li>
@@ -99,7 +97,7 @@
       </description>
     </release>
     <release version="0.3.1" date="2022-06-04">
-      <description>
+      <description translatable="no">
         <ul>
           <li>Translation updates</li>
           <li>Remove release notes dialog</li>
@@ -109,7 +107,7 @@
       </description>
     </release>
     <release version="0.3.0" date="2022-03-21">
-      <description>
+      <description translatable="no">
         <p>The second feature update to extension-manager. Highlights include:</p>
         <ul>
           <li>View comments and reviews</li>
@@ -125,27 +123,27 @@
       </description>
     </release>
     <release version="0.2.3" date="2022-02-21">
-      <description>
+      <description translatable="no">
         <ul>
           <li>Translation updates</li>
         </ul>
       </description>
     </release>
     <release version="0.2.2" date="2022-02-02">
-      <description>
+      <description translatable="no">
         <ul>
           <li>Translation updates</li>
           <li>Fix special characters in search results</li>
         </ul>
       </description>
     </release>
     <release version="0.2.1" date="2022-01-24">
-      <description>
+      <description translatable="no">
         <p>Fixes a crash that sometimes occurs while uninstalling an extension</p>
       </description>
     </release>
     <release version="0.2.0" date="2022-01-24">
-      <description>
+      <description translatable="no">
         <p>The first feature update to extension-manager. Highlights include:</p>
         <ul>
           <li>Dark theme and support for overriding the system colour scheme</li>