Support for sending server ACLs

The other half of https://github.com/vector-im/riot-web/issues/6987

Currently people have to use devtools or outside scripts to send ACL events. Would be nice to have a way to send the ACLs from within Riot, hopefully reducing mistakes in sending the event. The most common mistake so far seems to be forgetting to include an allow rule.

This probably doesn't have to be a prominent "ban server" button, and can probably be done through commands:
* `/allow-server *.matrix.org example.org` - Adds `*.matrix.org` and `example.org` to the ACL
* `/block-server *.matrix.org example.org` - Adds `*.matrix.org` and `example.org` to the ACL, ensuring the allow rules still permit access to the room

Possibly the commands should also calculate the new rules and require confirmation via a dialog? This could be a diff of some kind, maybe showing which users would be affected. Could also put a little warning triangle on servers in the room that won't respect the ACL. The rules could also be squashed where possible to ensure that they are as simple as possible (ie: `*.matrix.org` collapses `test.matrix.org` and `test2.matrix.org` if they already exist).

Another possible solution is to just have a `/server-acls` command that brings up a dialog with checkboxes to blacklist/whitelist servers. This would also allow someone to see what they're doing in realtime. Same sort of warning triangles for old servers would be nice, as well as the affected users.