[FEAT] Show follow symlink button

- When a user goes opens a symlink file in Forgejo, the file would be
rendered with the path of the symlink as content.
- Add a button that is shown when the user opens a *valid* symlink file,
which means that the symlink must have an valid path to an existent
file and after 999 follows isn't a symlink anymore.
- Return the relative path from the `FollowLink` functions, because Git
really doesn't want to tell where an file is located based on the blob ID.
- Adds integration tests.

templates/repo/view_file.tmpl

@@ -43,6 +43,9 @@
 			{{end}}
 			{{if not .ReadmeInList}}
 				<div class="ui buttons gt-mr-2">
+					{{if .SymlinkURL}}
+						<a class="ui mini basic button" href="{{$.SymlinkURL}}" data-kind="follow-symlink">{{ctx.Locale.Tr "repo.file_follow"}}</a>
+					{{end}}
 					<a class="ui mini basic button" href="{{$.RawFileLink}}">{{ctx.Locale.Tr "repo.file_raw"}}</a>
 					{{if not .IsViewCommit}}
 						<a class="ui mini basic button" href="{{.RepoLink}}/src/commit/{{PathEscape .CommitID}}/{{PathEscapeSegments .TreePath}}">{{ctx.Locale.Tr "repo.file_permalink"}}</a>