Since the frontend for this feature isn't ready, better to hide the
confusing field behind a feature flag.

This can be done trough the API for the current user, or on the
profile page.

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

If using gravatar.com, both plain_url and ssl_url in settings are now
served via HTTPS. Make this the default.

[ci skip]

Closes #20918

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

See merge request !2071

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

- Some views had a "Close" button. We've removed this, because we don't
  want users accidentally hiding the validation errors and not knowing
  what needs to be fixed.
- Some views used `li`, some used `p`, some used `span`. We've
  standardized on `li`.
- Some views only showed the first error. We've standardized on showing
  all of them.
- Some views added an `#error_explanation` div, which we've made
  standard.

This reverts commit 01160fc0, reversing
changes made to 4bff9daf.

Closes #13853