[keyserver] Introduce updateUserAvatar updater and use in updateUserAvatarResponder
Summary: updateUserAvatar sets the avatar column of the users table for a given viewer with the contents of the UpdateUserAvatarRequest we receive from updateUserAvatarResponder... unless we receive { +type: 'remove' }, in which case we set the avatar column to null.
Test Plan:
Use the following on native to send requests to keyserver:
- Observe that avatar column is empty for user (atul-web):
- Send request of type: 'emoji' and observe that column is populated:
- Send request of type: 'remove' and observe that column is empty again:
Reviewers: ashoat, ginsu
Reviewed By: ginsu
Subscribers: tomek
Differential Revision: https://phab.comm.dev/D7152