This essentially allows to pass both user and email, so that we can
either prefer the user to retrieve the avatar or (if user is not
present) fall back to the email lookup.

When we don't use the original `ApplicationHelper#avatar_icon` anymore, we
can just remove it (and its specs).

Closes #42800.

Callers expect a URL.

This fixes the avatars in commit lists when a user has no avatar and
Gravatar is disabled.

Closes #38715

The locale code is turned into an asset path with an underscore
instead of a dash.

The language codes are transformed by `I18n.locale` into a code with a
dash. But the resources for translating are always stored in a path
using a `_` separating the language and the region code.

The initializers including this were doing so at the top level, so every object
loaded after them had a `current_application_settings` method. However, if
someone had rack-attack enabled (which was loaded before these initializers), it
would try to load the API, and fail, because `Gitlab::CurrentSettings` didn't
have that method.

To fix this:

1. Don't include `Gitlab::CurrentSettings` at the top level. We do not need
   `Object.new.current_application_settings` to work.
2. Make `Gitlab::CurrentSettings` explicitly `extend self`, as we already use it
   like that in several places.
3. Change the initializers to use that new form.

only display the toggle setting in development env for now

Start of the new navigation by redesigning just the top navigation menu.
This is only shown when a cookie is set.

Part of #32794

Let the backend do the translation.

Instead of the JS being in charge of the HTML, the HAML now handles it.
The HAML can then check the cookie & show it needed. It also allows the
HAML access to the paths so we don't have to pass that through.

Closes #29955