[native][web] Flip the switch for blob-hosted media/avatar uploads

Summary:
Flip the switch for enabling blob-hosted multimedia.

Depends on D13890

Test Plan:
Verified in MariaDB that new uploads have empty content column and instead have the (blob hash, holder) pair.
Checked native/web, both avatars and multimedia meessages.

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D13891