Support adding/removing a reaction

[gnprice]

This will be a followup to:

• Show message reactions #121

This issue is for adding UI to support the user adding or removing a reaction of their own to a message.

For this issue, it's enough if the user can:

• add a thumbs-up reaction;
• remove a reaction they've already made; and
• join in on an existing reaction other people have made.

For Beta 2 we'll want a further ability, which we'll leave as a followup issue:

• Support adding an arbitrary reaction #388

[chrisbobbe]

add a thumbs-up reaction;

At first, this can just be an item in the message action sheet. Then later we can add the ability to add an arbitrary reaction.