[terraform] Create module for webapp/landing and convert webapp to module
Summary:
This implements a module for the webapp and replaces the self-host webapp to use it
Depends on D12927
Test Plan: terraform apply. Webapp was accessible as prior to module implementation
Reviewers: varun, bartek
Reviewed By: bartek
Subscribers: ashoat, tomek
Differential Revision: https://phab.comm.dev/D12906