Add Discord Plugin that uses Interaction API

[gigxz]

Create a Discord Plugin

• One Plugin instance is connected to one Guild. The community_platform_id is the guild ID.
• Supports OAuth install and OAuth user login.
  • User IDs in discord are unique across discord, so its possible that User 1234 belongs to multiple Discord guilds, several of which could have the Metagov bot installed. Unlike Slack, when a user logs in with Discord Oauth they just log in as themselves, the don't select a guild to "log in with." That means during user login, there could be multiple Discord Plugin instances that the user "belongs" to. We return the guild IDs of all metagov-enabled guilds back to the Driver after oauth. In the PolicyKit case, if there are multiple PolciyKit-enabled guilds, it displays a guild selection screen before logging in to PolicyKit.
• Supported actions
  • register-guild-command to create a slash command (aka CHAT_INPUT Application Command) in the guild. Later on, we might want to support global commands.
  • method to invoke any discord API method
  • post-message to post a message in a channel
  • get-guild to get guild info
• "listens" for Slash command input from users, and passes it along to the driver.
• vote process supports boolean and choice voting, with eligible voter restrictions. Voting occurs through interactive message components. Most of this code is copy-pasted from the Slack plugin, so there is room to DRY this code up later.