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/components/shared/plugin-common.js

@@ -1,7 +1,7 @@
-// Shared helpers for the plugin pages (`plugins-page`, `plugin-catalog`,
-// `plugin-detail`). Kept separate from `connector-common.js` on purpose: the
-// plugin model (JSON-Schema config blobs, `plugin_access`) is not the
-// connector model (env/api_key manifests).
+// Shared helpers for the plugin pages (`plugin-catalog`, `plugin-detail`).
+// Kept separate from `connector-common.js` on purpose: the plugin model
+// (JSON-Schema config blobs, `plugin_access`) is not the connector model
+// (env/api_key manifests).
 
 export async function jf(url, opts) {
   const res = await fetch(url, opts);