Chatmail: Sign-up webpage

[s0ph0s-dog]

This is part of a series of issues tracking chatmail support in Mox.

Chatmail servers have a sign-up webpage that provides basic information about a server, such as who runs it, what rules they have established, and a button + QR code that provide a dcaccount:https://chatmail.example/new link for creating an account. The /new endpoint should accept POST requests and return a JSON object:

{
    "email": "random@chatmail.example",
    "password": "random",
}

The local part of the email address should be 9–12 cryptographically random numbers and letters. The password should be 16+ cryptographically random numbers, letters, and symbols. Delta Chat clients will ask this endpoint for a username and password to use—but note that the server-side account creation should happen on first log in, not when the HTTP request happens!

In keeping with the established (minimalist) design language, the index page should use styles sparingly.

• Add new WebChatmail handler which produces the index page and /new endpoint
• Add configuration options to the new handler which allow administrator-defined text to be inserted into the index page: a blurb describing the server's goals, a list of rules/policies, and contact information for the administrator
• Generate a QR code with the dcaccount: link
• Test to ensure that a WebStatic handler for the index page can override the WebChatmail index page, in the event that the server administrator wishes to provide their own information page