0.18.0

Synced to flipcash2-protobuf-api@9d44350b,
picking up #135,
#136,
#137 and
#138. Only chat.v1 moved;
blob.v1 changed only in comments.

Renumbered

• EditChatResponse.Result: COVER_PICTURE_BLOB_NOT_ACCEPTED is inserted at 5, and
  DESCRIPTION_MODERATED moves from 5 to 6. 0.17.0 shipped DESCRIPTION_MODERATED = 5, so a
  0.17.0 client reads a cover-picture rejection from a current server as a moderated description,
  and does not recognize 6 at all. Code that maps this enum by raw value, such as iOS's
  Error*(rawValue:), has to be re-checked case by case, not only extended.

No other field or enum moved. StartChatResponse.Result gains its new case at the end.

Renamed

The group picture becomes the profile picture, to tell it apart from the new cover picture. Field
numbers are unchanged, so this is wire-compatible but breaks source in both languages.

• Metadata.picture → profile_picture (= 9).
• picture → profile_picture (= 2) on PublicGroupChatParameters and
  PrivateGroupChatParameters.
• EditChatRequest.picture → profile_picture (= 3), and its type Picture → ProfilePicture.
• MetadataUpdate.picture_changed → profile_picture_changed (= 5), and PictureChanged →
  ProfilePictureChanged with new_picture → new_profile_picture.
• PICTURE_BLOB_NOT_ACCEPTED → PROFILE_PICTURE_BLOB_NOT_ACCEPTED on StartChatResponse.Result
  (= 3) and EditChatResponse.Result (= 4).

Added

• Group cover pictures, the banner behind a chat's profile view.
  • Metadata.cover_picture (= 17). The feed RPCs may leave it unset even when one is set, so a
    feed result must not clear a cover picture the client already holds; GetChat returns it.
  • Set at creation through cover_picture on PublicGroupChatParameters (= 5) and
    PrivateGroupChatParameters (= 4), and changed through EditChatRequest.cover_picture
    (= 5, a CoverPicture { blob_id } wrapper).
  • Announced as MetadataUpdate.CoverPictureChanged (= 7, new_cover_picture).
  • COVER_PICTURE_BLOB_NOT_ACCEPTED on StartChatResponse.Result (= 7) and
    EditChatResponse.Result (= 5, see above).
• RosterUpdate.MembershipChanged (= 3), an empty case for a roster change the recipient is not
  shown. Apply its roster_summary by version and leave the cached member list alone.
• Chat.SampleChatters, a public sample of up to 100 members of a public group: its creator, then
  the most recent senders. Auth is optional. Returns SampledChatter (user_profile,
  last_sent_at, is_creator) and has_more.
• Chat.SetFeaturedGroups and Chat.GetFeaturedGroups, an ordered list of up to 10 public groups a
  user shows on their profile. GetFeaturedGroups looks the user up by username, takes optional
  auth, and returns each group as a list view shows it, without cover_picture or any viewer or
  messaging state.

Changed behavior

• GetRoster now requires membership. A non-member whom a group's listener rules let read its
  messages used to get the roster and now gets DENIED. SampleChatters is the public
  alternative.

Upgrading

Expect compile errors from the renames. Beyond those, an exhaustive MetadataUpdate switch needs
CoverPictureChanged, an exhaustive RosterUpdate switch needs MembershipChanged, and anything
that calls GetRoster for a group the user has not joined needs to stop.