More command parameter autocompletions

* [ ] One or more user IDs in `/pm`
* [ ] User ID as the first parameter in `/kick`, `/ban`
* [ ] Currently banned user IDs in `/unban`
* [ ] Known event types as the second parameter of `/sendevent` and `/setstate` and first parameter of `/msendevent` and `/msetstate`
* [ ] Room IDs as the first parameter of `/sendevent` and `/setstate`
* [ ] Existing toggles in `/toggle`