Allow system admins to send server notices to all local users

This issue has been migrated from [#5501](https://github.com/matrix-org/synapse/issues/5501).

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(It seems that there have been previous issues to track this, but all appeared to be closed at the moment)

Admin APIs exist to send server notices to users - https://github.com/matrix-org/synapse/blob/master/docs/admin_api/server_notices.md (, and https://github.com/matrix-org/synapse/blob/master/docs/server_notices.md for more generic info. on server notices and their expected use). However, at present it only allows for messages to be sent to specific users.

It would be very useful to be able to send messages to all local users (e.g. for "Messages of the day" or policy updates etc.).

Can we please open up the existing API to send to all users matching a pattern (including '.*'), or create a new endpoint to allow for this?

Read docs/admin_api/server_notices.md and docs/server_notices.md first to understand the existing server-notice API and targeting model. Then trace the existing admin API entry point for sending notices; done means an authenticated admin can send one notice to all local users or a documented matching pattern, with tests covering the behavior.

Written by the indexing model from the issue text.