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"

routers/web/repo/wiki.go

@@ -238,7 +238,7 @@ func renderViewPage(ctx *context.Context) (*git.Repository, *git.TreeEntry) {
 		ctx.Redirect(ctx.Repo.RepoLink + "/wiki/?action=_pages")
 	}
 	if isRaw {
-		ctx.Redirect(util.URLJoin(ctx.Repo.RepoLink, "wiki/raw", string(pageName)))
+		ctx.Redirect(ctx.Repo.RepoLink + "/wiki/raw/" + string(pageName))
 	}
 	if entry == nil || ctx.Written() {
 		return nil, nil