feat(upload): avatar and calendar cover via local disk storage.

POST /v1/user/me/avatar and /v1/calendars/:id/cover; GET /v1/media;
UPLOAD_DIR on eventhub-data volume; MIME/size limits.
Refs EventHub/EventHubBack#71
Refs EventHub/EventHubSpec#8

docker/docker-compose.stage.yml

@@ -49,6 +49,8 @@ services:
       - SMTP_TLS=${SMTP_TLS:-if_available}
       - PUBLIC_APP_URL=${PUBLIC_APP_URL:-https://stage.calentiq.com}
       - REMINDER_LEAD_HOURS=${REMINDER_LEAD_HOURS:-24}
+      - UPLOAD_DIR=${UPLOAD_DIR:-/app/data/uploads}
+      - UPLOAD_MAX_BYTES=${UPLOAD_MAX_BYTES:-2097152}
     networks:
       eventhub-net:
         aliases: