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.

templates/repo/view_file.tmpl

@@ -43,7 +43,9 @@
 				<a download href="{{$.RawFileLink}}"><span class="btn-octicon" data-tooltip-content="{{ctx.Locale.Tr "repo.download_file"}}">{{svg "octicon-download"}}</span></a>
 				<a id="copy-content" class="btn-octicon {{if not .CanCopyContent}} disabled{{end}}"{{if or .IsImageFile (and .HasSourceRenderedToggle (not .IsDisplayingSource))}} data-link="{{$.RawFileLink}}"{{end}} data-tooltip-content="{{if .CanCopyContent}}{{ctx.Locale.Tr "copy_content"}}{{else}}{{ctx.Locale.Tr "copy_type_unsupported"}}{{end}}">{{svg "octicon-copy" 14}}</a>
 				{{if .EnableFeed}}
-				<a class="btn-octicon" href="{{$.FeedURL}}/rss/{{$.BranchNameSubURL}}/{{PathEscapeSegments .TreePath}}">{{svg "octicon-rss" 14}}</a>
+				<a class="btn-octicon" href="{{$.FeedURL}}/rss/{{$.BranchNameSubURL}}/{{PathEscapeSegments .TreePath}}" data-tooltip-content="{{ctx.Locale.Tr "rss_feed"}}">
+					{{svg "octicon-rss" 14}}
+				</a>
 				{{end}}
 				{{if .Repository.CanEnableEditor}}
 					{{if .CanEditFile}}