[native][web] Add migration to unshim Blob-hosted multimedia
Summary:
Point 3c from ENG-6459.
Depends on D13888
Test Plan:
- Before applying D13888 and this, I sent some encrypted multimedia messages. They appeared as robotext
- Applied both diffs and ran migration
- Multimedia messages appeared normally
Reviewers: ashoat, tomek
Reviewed By: ashoat, tomek
Subscribers: tomek
Differential Revision: https://phab.comm.dev/D13889