[web] Terms of Use & Privacy modal

Summary:
Modal showing Terms of Use and Privacy Policy acknowledgment.

Design:

Example after server error

Test Plan:

1. I accept should update the value on keyserver DB
2. User should not be able to close the modal using any method

Reviewers: tomek, atul, ashoat

Reviewed By: tomek, ashoat

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D5829