[web] Add backup testing screen

Summary:
Mentioned in D11043, this diffs adds a test backup restoration screen (staff only) to the web that can be used to trigger the backup restore message with the specfic backupID and encryption keys. The styling is taken from the tunnelbroker test screen.

Depnds on D11074

Test Plan:

• Update the code so that the accessToken is an empty string and not null
• Create a backup on native and log backupID and encryption keys
• Open the new web modal and run restoration
• Check if restoration completed successfuly

Reviewers: marcin, bartek, kamil

Reviewed By: kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11142