[web] Import backup client

Summary:
Actually use the WASM backup client introduced in the previous diff.

• webpack/keyserver machinery for downloading the wasm file
• updated the worker init message so that it also intialized the backup client

Depends on D11041

Test Plan:
Test on both:

• yarn dev in web and keyserver
• yarn prod, yarn prod-build

Send the worker init message, tested that there were no errors, and the WASM was correctly downloaded.

Reviewers: marcin, bartek, kamil

Reviewed By: kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11042