Fix URLJoin, markup render link reoslving, sign-in/up/linkaccount page common data (#36861)

The logic of "URLJoin" is unclear and it is often abused.

Also:
* Correct the `resolveLinkRelative` behavior
* Fix missing "PathEscape" in `ToTag`
* Fix more FIXMEs, and add new FIXMEs for newly found problems
* Refactor "auth page common template data"

services/wiki/wiki_path.go

@@ -165,7 +165,7 @@ func ToWikiPageMetaData(wikiName WebPath, lastCommit *git.Commit, repo *repo_mod
 	_, title := WebPathToUserTitle(wikiName)
 	return &api.WikiPageMetaData{
 		Title:      title,
-		HTMLURL:    util.URLJoin(repo.HTMLURL(), "wiki", subURL),
+		HTMLURL:    repo.HTMLURL() + "/wiki/" + subURL,
 		SubURL:     subURL,
 		LastCommit: convert.ToWikiCommit(lastCommit),
 	}