Allow creation of OAuth2 applications for orgs (#18084)

Adds the settings pages to create OAuth2 apps also to the org settings
and allows to create apps for orgs.

Refactoring: the oauth2 related templates are shared for
instance-wide/org/user, and the backend code uses `OAuth2CommonHandlers`
to share code for instance-wide/org/user.

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

web_src/less/_base.less

@@ -1804,7 +1804,9 @@ a.ui.label:hover {
   border: 1px solid var(--color-light-border);
   color: var(--color-text);
 }
-
+.ui.tertiary.button {
+  border: none;
+}
 .page-content .ui.button {
   box-shadow: none !important;
 }