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.

Cargo.lock

@@ -3010,6 +3010,7 @@ version = "0.1.0"
 dependencies = [
  "anyhow",
  "async-trait",
+ "axum",
  "chrono",
  "core-api",
  "rand 0.10.1",