0.12.0 Update `web-app` version. Add `github_webhooks_token`. Pin `aws` provider

what

• Update web-app version
• Add github_webhooks_token
• Pin aws provider

why

• cloudposse/terraform-aws-ecs-web-app#34
• Read the GitHub webhooks token from SSM Parameter Store (similar to atlantis_gh_token). GitHub provider does not accept an empty token (https://github.com/cloudposse/terraform-github-repository-webhooks/blob/master/main.tf#L2). It should either be provided or the attribute omitted completely if the token is needed to be sourced from the GITHUB_TOKEN env variable
• Consistency, reproducibility