Strengthens validation requirements for group avatars a user sets. Also,
checks these requirements on avatars we receive.
Introduces a data migration to drop any large group avatars.
This fixes an issue some users were reporting of jetsaming in the share
extension. All group avatars are stored in-memory on TSGroupModel.
Until we have a better strategy for memory management here, we'll want
to keep group avatars small.
34e0f99cfd
