When creating a new application, give user chance to supply a VPC ID
If VPC ID is supplied, record it in the db (schema change - should probably add this in any case and record the VPC ID when creating a new app the status quo way), and skip creating it (requires change to Terraform config/order of operations/supplying Terraform with variables, also should hit AWS API to confirm it exists)
Separately but related, allow the user to supply AWS creds such that we can call AWS API on their behalf to get a list of VPCs to populate the front-end (alternative to them typing in a VPC ID), see issue https://github.com/adhocteam/soapbox/issues/152