[web] Rename multimedia holder to blobURI

Summary:
Part of ENG-3975.
The same as D8323, but for web - Now the holder isn't accurate, I decided to name this value blobURI because it is of format: either https://keyserver/uploads/... or blob-service://blob-hash

Depends on D8322

Test Plan: Flow, also ran the web locally to see if encrypted media are displayed correctly

Reviewers: atul, ashoat

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D8325