Server notices feature is not provisioner-friendly

This issue has been migrated from [#5535](https://github.com/matrix-org/synapse/issues/5535).

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**Description:**

As brought up in https://github.com/matrix-org/matrix-hosted/issues/709#issuecomment-504879001, there's currently no way to set a custom message for the room except for the consent message. Additionally, it's possible for the user to be invited to the room and find no message waiting for them (problem discussed in the above issue), which can be really confusing first impressions.

Here are the problems with the feature I've identified:

- [ ] The user can be invited to the Server Notices room even if there isn't a message waiting for them.
- [ ] A custom message cannot be specified except for one requiring consent. Ideally another field would be added [in the config](https://github.com/matrix-org/synapse/blob/4f68188d0bbdb1966250375d34125572eb82a117/docs/sample_config.yaml#L1288) to set this.
- [ ] The sample config state that the Server Notices room is "a special room which cannot be left": https://github.com/matrix-org/synapse/blob/4f68188d0bbdb1966250375d34125572eb82a117/docs/sample_config.yaml#L1274 Whereas our documentation states that "having joined the room, the user can leave the room if they want": https://github.com/matrix-org/synapse/blob/master/docs/server_notices.md#user-experience