[keyserver] Update uploads table type

Summary:
Currently user ID can be a UUID string, and we're using it as a container for avatars, so we have to update the type of container to match the one of the user ID.

https://linear.app/comm/issue/ENG-7316/unable-to-update-the-avatar#comment-5e10bd92

Test Plan: Check if it is possible to update an avatar (by changing an emoji avatar's background) and to send a message with an image.

Reviewers: inka, varun, kamil, ashoat

Reviewed By: ashoat

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D11838