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/css/page-shell.css

@@ -77,7 +77,6 @@ roles-page,
 shared-folders-page,
 connectors-page,
 connector-detail-page,
-plugins-page,
 plugin-catalog-page,
 plugin-detail-page,
 plugin-page-host,