Blocking a room leads to stuck invites

This issue has been migrated from [#11442](https://github.com/matrix-org/synapse/issues/11442).

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### Description

When banning a room using the admin API, multiple clients (Element Android and Nheko at least) experience issues declining the invite.

### Steps to reproduce

- Get invited to room (over federation maybe?)
- Ban that room from your HS
- Try declining that invite in Element Android or other clients

The server will return an error code when declining the invite, which in many clients leads to the invite being stuck. While clients could handle that better, I think synapse could help them out here. Especially since you otherwise need to decline the invite in all of your 16 logins.

Ideally the server would either accept the leave event and return the removed room on /sync, even when the room is banned from the HS or it would just send a leave event also to invited users when banning a room.

### Version information

- **Homeserver**: neko.dev, nheko.im

If not matrix.org:

- **Version**: 1.47.1

- **Install method**: ebuild

- **Platform**: Gentoo