Delete mosaics

With the introduction of `POST /mosaic` it can be expected that some created mosaics might not be used. For example, the user might change their mind on the mosaic dates before running a prediction, leaving an unused mosaic behind.

I believe we need a `DELETE /mosaic/:id` endpoint that only admins can call and that returns an error if there is any timeframes using that mosaic.

Another useful endpoint would be something like `POST /mosaic/purge` that delete all unused mosaics.

This is not blocking any feature now; it is just a suggested enhancement.

@geohacker @ingalls @LanesGood please let me know if you have thoughts.