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/editor.go

@@ -218,7 +218,8 @@ func redirectForCommitChoice[T any](ctx *context.Context, parsed *preparedEditor
 	}
 
 	// redirect to the newly updated file
-	redirectTo := util.URLJoin(ctx.Repo.RepoLink, "src/branch", util.PathEscapeSegments(parsed.NewBranchName), util.PathEscapeSegments(treePath))
+	redirectTo := ctx.Repo.RepoLink + "/src/branch/" + util.PathEscapeSegments(parsed.NewBranchName) + "/" + util.PathEscapeSegments(treePath)
+	redirectTo = strings.TrimSuffix(redirectTo, "/")
 	ctx.JSONRedirect(redirectTo)
 }