Add Reports to Admin API

Resolves #774 (closed)

• Get all reports
• Get filtered reports by status (resolved/unresolved)
• Get individual report by id
• Responding to a report
• Closing a report
• The ability to delete or change the scope of individual reported statuses (if any).

Notes:

The Responding to a report feature was implemented not as expected. Reports (Flag) activities don't have objects, so we can't put their id to inReplyTo field of responses (this line raises an error). I create a simple direct message from admin to a reporter instead.