Add "Add Reviewer" and "Assign" buttons to PR handlers

[russmiles]

At the moment we display a "Merge" button, and have an associated command for that. In line with the buttons available on an Issue, for a PR it would be good to display "Add Reviewer" and "Assign" buttons and have accompanying commands.

[kipz]

Currently the styling for these is done in the bot, but once this moves in to mustache/ts, this'll be easy!

[tanya-coding]

Handlers can add actions without changing anything in bot. Buttons are fully controlled by the handler and bot just displays all it receives so technically this is just a matter of updating PR push event handler