Improve RSS feed icons (#28368)

Improve RSS feed icons (#28368)

- The RSS Feed icons were placed in a proper button, so that it does
  not look "inconsistent". This also makes the problem of the button
  being improperly aligned go away.
- The icon that shows on user profiles has not been modified because
  of a lack of better implementation ideas.
- Where applicable, the RSS Feed icon was put directly next to the
  Follow button (right menu), as both functionalities effectively
  share the same purpose.
- Despite the attempt at achieving less inconsistency, a conscious
  decision to not add any text to those buttons was made, opting for
  tooltips instead. "Make it present, but not too annoying."
- A special exception was made for the Releases pages (which contains
  text, not a tooltip), where an RSS feed would be particularly
  beneficial to users.

The fact that the RSS functionality is explicitly optional was taken
into account, and these improvements were made with public-facing
instances (where the feature works best) in mind.