Move 3rd party js/css into public/vendor and document sources (#2383)

This commit cherry picks the following commits (master -> v1.2):
  a915a09
  874f9be
  233bbcf

Ref: https://github.com/go-gitea/gitea/issues/1484
Signed-off-by: Michael Lustfield <michael@lustfield.net>

integrations/links_test.go

@@ -28,7 +28,7 @@ func TestLinksNoLogin(t *testing.T) {
 		"/user/forgot_password",
 		"/swagger",
 		// TODO: follow this page and test every link
-		"/assets/librejs/librejs.html",
+		"/vendor/librejs.html",
 	}
 
 	for _, link := range links {