[keyserver] add entry in acknowledgment table on user sign up
Summary:
Context: ENG-1715
Users sign up means that terms of use and privacy policy was accepted, that's why entry in policy_acknowledgments should be added
Test Plan:
- register in mobile app
- check if entry to table was added
Reviewers: marcin, tomek, atul
Reviewed By: tomek, atul
Subscribers: ashoat, atul, abosh
Differential Revision: https://phab.comm.dev/D5664