[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.

modules/base/tool.go

@@ -174,7 +174,7 @@ func Int64sToStrings(ints []int64) []string {
 func EntryIcon(entry *git.TreeEntry) string {
 	switch {
 	case entry.IsLink():
-		te, err := entry.FollowLink()
+		te, _, err := entry.FollowLink()
 		if err != nil {
 			log.Debug(err.Error())
 			return "file-symlink-file"