Announcement clients

[pleary]

See inaturalist/inaturalist#3909. Should be deployed only after that PR is merged.

• Announcements endpoint accepts a client parameter and filters announcement accordingly
  • Announcements must include that client as a target, or must not target any clients
• When the client parameter is not specified, the request user-agent is used in attempting to auto-detect the relevant announcement client
• Announcement clients are part of the response