Generator for `_nav.html.erb`; automatically add tabs to `_nav.html.erb` if it exists in the namespace

• Nav templates (_nav.html.erb) are now automatically appended to if they exist. Remember nav template live in the views folder at the root of the namespace, which is one folder up from whatever folder is being built.
If a file exists _nav.html.erb, it will get appnded to with content like this:

<li class="nav-item">
    <%= link_to "Domains", domains_path, class: "nav-link #{'active' if nav == 'domains'}" %>
  </li>

This append to the _nav.html.erb template happens in addition to the partial itself being included from the layout.
(Also only if it exists, so be sure create it before running the scaffold generators for the namespace. Of course, you only need to create it once for each namespace)

• To create a new _nav.html.erb template use

bin/rails generate hot_glue:nav_template --namespace=xyz

Here, you give only the namespace. It will create an empty nav template:

<ul class='nav nav-tabs'>
</ul>

• Fixes to specs for datetimes

• Fixes way the flash notices where created that violated frozen string literal