[terraform] webapp terraform config

Summary:
The terraform configuration for running webapp nodes. Sets up a load balancer and a webapp ecs keyserver cluster service

Depends on D12813

Test Plan: Was able to access the web app through specified domain (comm.software)

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: bartek, tomek

Differential Revision: https://phab.comm.dev/D12814