Update Guild and Message Models

[Chris-Johnston]

This PR includes many updates to the Guild and Message API models and their related classes.
Fixes #1113 .

Guild:

• Adds the ExplicitContentFilter property and ExplicitContentFilterLevel type
  • Add a test for this property's ModifyAsync behavior
• Adds the ApplicationID property.
  • Can test this by having a bot create a new guild, then checking that the ApplicationId property is not null. This is not viable to include as part of the bot's integration tests in CI, as bots can only create up to 10 guilds iirc.

Message:

• Adds the Activity property and MessageActivity, MessageActivityType types
  • Can test this by inviting a channel w/ bot to listen along on Spotify
• Adds the Application property and the MessageApplication type
  • Can test this (and Activity) by inviting a channel w/ bot to a Fortnite lobby (I actually had to install the game to test this behavior)
    

This commit log may look a bit strange because of an issue when resolving merge conflicts.