* DB migration adds app_versions table for managing known app versions.
* /ping reads app version from User-Agent header and responds with latest version if the app is OKO and its version is older than the latest one stored.
* Added endpoint /app-versions which
* lists all known versions via GET
* adds/updates a version via POST
#3Fix#4
* photo metedata provided with downloaded data
* data download recognizes application/json and application/zip accepted types and serves bare json and zip with photos respectively
* thumbnail handling and storage
* errors put into separate package
* added table to store feature photos
* added API endpoints for serving photos
* POST /data extended to accept photos using a multipart/form-data body