Write custom levels to individual json-formatted .level files

Instead of writing a database migration task, I intend to migrate the existing level set directly from the level-builder data as follows:

1.  Merge this feature into levelbuilder first
1.  Generate the initial set of custom level files from the server data
1.  Delete old custom_levels.json
1.  Commit and push file changes
1.  Merge this feature plus the file-change commits together into finished