[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.

options/locale/locale_en-US.ini

@@ -1205,6 +1205,7 @@ tag = Tag
 released_this = released this
 file.title = %s at %s
 file_raw = Raw
+file_follow = Follow Symlink
 file_history = History
 file_view_source = View Source
 file_view_rendered = View Rendered