minor: ActivityQueueService

Closes #249 (closed) Closes #234 (closed)

Documentation here: https://git.startinblox.com/djangoldp-packages/djangoldp/wikis/guides/federation#activityqueueservice

• activity resend/reschedule attempts
• rejecting duplicate activities
• rejecting updates when nothing has changed
• preventing activity loops
• unit tests
• if the server is restarted, recover the scheduled activities on the queue
• thread-safe queue usage

Backwards-compatible changes to the Activity model, added a new ScheduledActivity subclass (which allows us to revive the queue on start-up)

Testing

• the unit tests I've added can be seen in the code changes
• tested the retries logic manually on a local server
• tested the startup code manually on a local server