[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