Whenever we already deal with a User object, let's use the more specific
method avatar_icon_for_user.

Tests for these new helper methods

+ Add a partial for displaying user deletion guidance

Add CHANGELOG entry

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

Closes #28842

Adds rel="noopener noreferrer" to all links with target="_blank"

See merge request !2071

Now it appends a template instead of relying in a partial
to show it.

Also changed the cookie sets to true and false values to make
it consistent with the rest of the javascript.

Fixed some haml and a missing new line

Also improved accesibility and change the id from user_callouts to a class

Also changed the user_callout haml to a partial and
added the corresponding SVG icon