[web] Caclulate encrypted media sha256

Summary:
Web counterpart of D7647.
Sometimes encountered weird errors when creating the olm.Utility multiple times so decided to cache the instance and lazy-init Olm.

Due to ENG-3845 I decided to opt out of Olm here and used WebCrypto instead

Test Plan: Added console.log in encryptMedia() and sent encrypted photo. Verified that the hash is calculated

Reviewers: atul, ashoat

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7688