Reactions: Add emoji picker as a message action

[jryans]

Currently we're thinking we'll use https://github.com/missive/emoji-mart.

To open the emoji picker:

• Clicking (🙂+) reveals the emoji picker
• Clicking 'View all' reveals the emoji picker

[turt2live]

Would be great if it was added in such a way where matrix-org/matrix-spec-proposals#1959 could be supported in future.

Note from the proposal:

Although this proposal does not include an emoji picker definition, the API it proposes can easily and transparently be extended to such a widget.

[jryans]

Would be great if it was added in such a way where matrix-org/matrix-doc#1959 could be supported in future.

As fas as I can see, we should be able to the extend the picker component with custom data sets to achieve this. It may take some amount of customising / forking to do so, but I think that's acceptable.

[poperigby]

It would probably be a good idea to have the emoji picker in the right-click menu as well.