Add interactive buttons for webhooks

We'll approach implementation similar to Outlook's interactive messages https://docs.microsoft.com/en-us/outlook/actionable-messages/card-reference

They are simply better than what Slack, Facebook, HipChat and others offer. Much more configurable & secure, easy polling support, easily extended to additional interactive messages like menus, etc.

Implementation should include support with MultichoiceInput option (or something similar), to make polls easier.

Original description:

Per this UserVoice request.

The scope of this ticket is limited to Message Buttons - Message Menus will be investigated separately in future, as it will be a much larger task to fully implement.

Developing this feature will necessitate:

• Alterations to the configuration of Incoming Web Hooks (& potentially Slash Commands)

• Additional server side code to support the action call-outs.

• Additional API/App layer code to support the new integration message button presses.

• Additional UI layer code to support the display (and API calls back to the server) of the message buttons.

If you wish to take on this ticket, once you have determined how you would like to implement each part of it, please discuss your plans with the core developer team in the ~developers channel of our Mattermost server.