Add Get, Create, and Delete endpoints for Announcements

[blakestone95]

Description: We need server endpoints to retrieve and manipulate announcements (extension of #21).

Acceptance Criteria:

• Can get list of announcements
  • Order descending by post date
  • Accepts optional date range parameter (default to all dates)
  • Supports pagination:
  • Accepts optional "# per page" parameter (default to 10)
  • Accepts optional "page #" parameter (defaults to 1)
• Can get single announcement by ID
• Can create single announcement
  • ID generated server-side or on DB, client shouldn't have to worry about that
• Can modify single announcement by ID
• Can mass delete announcements
  • Accepts an array of ID's to cover both the single delete and multiple delete cases

[dakriy]

Done in #56