Per-room configurable quick reactions

Many bots are designed to perform actions when a particular emoji is used to react to a message in chat. (For example, the TWIM bot records a user's message for output in the TWIM blog post if it is reacted to with the hollow red circle emoji.)

Additionally, in other rooms, the culture of the room has given importance to specific emojis (for example :arrow_up: indicating agreement with the a message).

**Describe the solution you'd like**
It would be nice to be able to configure on a per-room basis the "Quick Reactions" that should be available. This will greatly enhance the experience on mobile clients where searching for emojis is more difficult than on desktop clients.

**Describe alternatives you've considered**
* An alternative would be to make this entirely client-side and allow users to "pin" specific emojis (either at a per-room level or globally). But this would require the user to perform some action rather than moderators being allowed to specify a set of easily-accessible emojis. Additionally, this wouldn't be cross client, which would mean that you would have to configure this on all of your devices.

Review issue #700 and the Matrix specification structure to determine where per-room quick-reaction configuration belongs. Define the protocol behavior and configuration semantics for room moderators, including how clients expose the configured reactions; done when the specification fully describes interoperable support.

Written by the indexing model from the issue text.