Apply config changes for CSP file uploads.

This applies configuration changes for the Flask app and adds changes to
the Dockerfile so that the build can make a CSP-specific JS bundle. It
adds `write_dotenv` script that creates the appropriate `.env` file for
the `parcel` bundler depending on how the `CSP` environment variable is
set.

- Configure K8s environment variables for Flask CSP usage
- Supply default CSP config setting to Flask app
- Declare the CSP arg in the Dockerfile
- Supply extra Docker build args to CD
- Fix top-level reference to boto3 in file_upload module
- Add back missing sample NGINX config for docker-compose build

.circleci/config.yml

@@ -129,6 +129,7 @@ workflows:
           requires:
             - app_setup
       - azure-acr/build_and_push_image:
+          extra-build-args: "--build-arg CSP=azure"
           login-server-name: "${AZURE_SERVER_NAME}"
           registry-name: pwatat
           repo: atat
@@ -175,6 +176,7 @@ workflows:
               only:
                 - master
       - aws-ecr/build_and_push_image:
+          extra-build-args: "--build-arg CSP=aws"
           repo: atat
           tag: "${CIRCLE_SHA1}"
           requires: