[web] policy acknowledgment handler

Summary: Code responsible for showing/hiding modal with policy acceptance. Currently, only with Terms of Use & Privacy because it's the only police we have right now.

Test Plan:

1. Log in to web app
2. Change confirmed field for logged user in policy table
3. Modal should appear (not immediately but after any request will be sent to keyserver)
4. I accept should close modal and update value on keyserver db
5. User should not be able to close modal using any method

Reviewers: tomek, atul

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D5830