Rich Announcements feature

[Gerzer]

We should implement a new Rich Announcements feature that enables rich content in announcements. The following types of rich content should be supported at minimum:

• Basic text formatting (perhaps with Markdown)
  • Bold
  • Italic
• Inline hyperlinks
  • https:
  • mailto:
• Attachments
  • Calendar events

Support for calendar events is particularly important because we’ll want to send out an announcement for the Shuttle Tracker Special Event that makes it easy for users to add the event to their personal calendars to be reminded to attend.

This will probably require a breaking change to the public API, so we should batch the change with #64 when we increment the API version number.