Implement basic participant controller functionality

[bunsenmcdubbs]

• Creating a new table for participant "flags"
  • via database migration
• Add new function(s) in model/participant/participant.go for:
  • setting flag(s)
    • Add unit test
  • getting flagged participants
    • Add unit test
  • getting all participants
    • Add unit test
• Adding a participant controller with routes for all of the above^ (remember to properly catch and return errors)

Relevant routes from #58 (to be finalized):

Route	Method	Description	Request	Response
/participant	GET	Get all participants	-	{participants: []<participant>}
/participant/flag	PUT	Flag participants for the current user	{participant_ids: []<id>}	{status: "ok"}
/participant/flag	DELETE	Unflag participants for the current user	{participant_ids: []<id>}	{status: "ok"}
/participant/flagged	GET	Get all flagged participants for the current user	-	{participants: []<participant>}
/participant/{id}/flag	POST	Flag participant for the current user	None	{status: "ok"}
/participant/{id}/flag	DELETE	Unflag participant for the current user	None	{status: "ok"}

Paths/interface will be finalized in #58

If #57 (user authentication middleware) is not yet complete, this can be initially implemented with a static user and wait until that feature is merged.