Add recipients field to activities.

Also do some very basic checks for AP message insertion.
1 job for feature/activitypub in 2 minutes and 23 seconds (queued for 4 seconds)
Name Stage Failure
failed
unit-testing Test There is an unknown failure, please try again
       test/web/activity_pub/activity_pub_controller_test.exs:36: (test)

.........

Finished in 30.1 seconds
318 tests, 4 failures

Randomized with seed 352998
ERROR: Job failed: exit code 1