plugins: merge the user Plugins page into per-plugin sidebar pages

The generic per-user #plugins page is gone: a plugin with per-user
settings hosts them in its own web_pages() sidebar page instead
(Telegram's pairing page is new; Honcho's opt-in page already existed).
The admin catalog moves from #plugin-catalog to #plugins (old hash
redirected), and user_config_schema is removed from the Plugin trait,
the API DTOs and both plugins — the my-config endpoint, the
plugin_user_configs store and the update_user_config hook stay, now
driven by each plugin's own page fragment.

web/index.html

@@ -96,7 +96,6 @@
       <shared-folders-page></shared-folders-page>
       <connectors-page></connectors-page>
       <connector-detail-page></connector-detail-page>
-      <plugins-page></plugins-page>
       <plugin-page-host></plugin-page-host>
       <plugin-catalog-page></plugin-catalog-page>
       <plugin-detail-page></plugin-detail-page>