Improve assets handler middleware (#15961)

* Use route to serve assets but not middleware

* Fix build error with bindata tag

* convert path to absolute

* fix build

* reduce function stack

* Add tests for assets

* Remove test for assets because they are not generated

* Use a http function to serve assets

* Still use middleware to serve assets then less middleware stack for assets

* Move serveContent to original position

* remove unnecessary blank line change

* Fix bug for /assets* requests

* clean code

Co-authored-by: zeripath <art27@cantab.net>

integrations/links_test.go

@@ -35,6 +35,8 @@ func TestLinksNoLogin(t *testing.T) {
 		"/user2/repo1",
 		"/user2/repo1/projects",
 		"/user2/repo1/projects/1",
+		"/assets/img/404.png",
+		"/assets/img/500.png",
 	}
 
 	for _, link := range links {