[GtS support] interaction policies

[connyduck]

The feature is explained here: https://docs.gotosocial.org/en/latest/user_guide/settings/#default-interaction-policies
The api definition is here: https://docs.gotosocial.org/en/latest/api/swagger/

So basically we need

• a new account preference page for default interaction policies
  • should only be visible on GtS instances
• A way to change the interaction policy in the compose screen before posting
  • An additional icon somwhere that opens a dialog for it?
  • Combine it with the visibility settings?
  • Should only be available on gts instances
• A way to see and handle interaction requests
  • I imagine this being quite similar to Mastodons notification policies
• Show the correct buttons on a post so users that are not allowed to interact can't actually interact
• Somehow display the interaction policy that is set on a post
  • probably in the detail view where we display other metadata