Support for resticting what size groups can be bridged to

[jaywink]

It would be nice to be able to configure an upper limit of how large the remote Telegram group can be for bridging to happen. This could be used by homeserver admins to ensure their users don't cause issues for the homeserver by bridging into groups of tens of thousands or more members.

The limit could either be applied on the first bridging attempt or enforced later.

[tulir]

Mostly done in https://github.com/mautrix/telegram/commit/e8eef1c31e4aaa282b79717a4f8399b5e9c94f9a, although it doesn't check the member count when bridging manually