Move Site Announcements off Stream

[NuckChorris]

What

• Create a SiteAnnouncementView table joining User with SiteAnnouncement and tracking read+seen status
  • The separate read+seen will allow us to show a different unread announcement each visit, by showing the least-recently-seen one!
• Expose the site announcement view through graphql

The next step will be rolling out a feature flag for client-side switching of the site announcement view to be backed by this

Why

• Stream costs a lot of money, we should reserve it for actual feeds
• Marking announcements as seen is broken for some reason
• It's just all kinds of janky to use feeds for this

Checklist

• All files pass Rubocop
• Any complex logic is commented
• Any new systems have thorough documentation
• Any user-facing changes are behind a feature flag (or: explain why they can't be)

• All the tests pass
• Tests have been added to cover the new code

[codeclimate]

Code Climate has analyzed commit 6459a31 and detected 2 issues on this pull request.

Here's the issue category breakdown:

Category	Count
Duplication	2

The test coverage on the diff in this pull request is 90.1% (80% is the threshold).

This pull request will bring the total coverage in the repository to 73.3%.

View more on Code Climate.