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"

modules/markup/html_mention.go

@@ -4,6 +4,7 @@
 package markup
 
 import (
+	"fmt"
 	"strings"
 
 	"code.gitea.io/gitea/modules/references"
@@ -26,14 +27,11 @@ func mentionProcessor(ctx *RenderContext, node *html.Node) {
 		loc.End += start
 		mention := node.Data[loc.Start:loc.End]
 		teams, ok := ctx.RenderOptions.Metas["teams"]
-		// FIXME: util.URLJoin may not be necessary here:
-		// - setting.AppURL is defined to have a terminal '/' so unless mention[1:]
-		// is an AppSubURL link we can probably fallback to concatenation.
-		// team mention should follow @orgName/teamName style
+
 		if ok && strings.Contains(mention, "/") {
 			mentionOrgAndTeam := strings.Split(mention, "/")
 			if mentionOrgAndTeam[0][1:] == ctx.RenderOptions.Metas["org"] && strings.Contains(teams, ","+strings.ToLower(mentionOrgAndTeam[1])+",") {
-				link := "/:root/" + util.URLJoin("org", ctx.RenderOptions.Metas["org"], "teams", mentionOrgAndTeam[1])
+				link := fmt.Sprintf("/:root/org/%s/teams/%s", ctx.RenderOptions.Metas["org"], mentionOrgAndTeam[1])
 				replaceContent(node, loc.Start, loc.End, createLink(ctx, link, mention, "" /*mention*/))
 				node = node.NextSibling.NextSibling
 				start = 0