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/context/repo.go

@@ -961,7 +961,8 @@ func RepoRefByType(detectRefType git.RefType) func(*Context) {
 				}
 				// If short commit ID add canonical link header
 				if len(refShortName) < ctx.Repo.GetObjectFormat().FullLength() {
-					canonicalURL := util.URLJoin(httplib.GuessCurrentAppURL(ctx), strings.Replace(ctx.Req.URL.RequestURI(), util.PathEscapeSegments(refShortName), url.PathEscape(ctx.Repo.Commit.ID.String()), 1))
+					// FIXME: the dirty hack of "strings.Replace" should be fixed
+					canonicalURL := strings.TrimSuffix(httplib.GuessCurrentAppURL(ctx), "/") + strings.Replace(ctx.Req.URL.RequestURI(), util.PathEscapeSegments(refShortName), url.PathEscape(ctx.Repo.Commit.ID.String()), 1)
 					ctx.RespHeader().Set("Link", fmt.Sprintf(`<%s>; rel="canonical"`, canonicalURL))
 				}
 			} else {