Add ability to remove modules from the storage

[marpio]

Is your feature request related to a problem? Please describe.
Even though athens storage should be immutable, there are some cases where modules will have to be deleted i.e. due to a DMCA infringement.

Describe the solution you'd like
The storages already implements the Deleter interface, what's missing is an http endpoint. It might be an manual process only possible from the UI and only accessible to users with a specific permission.

[arschles]

we'll need to have some authorization mechanism for deletes

[lopezator]

This would be possible now, although not ideal, by using "basic auth".

I agree that we need something to delete specific packages in case something goes bad (bad checksum, corrupted package or whatever).

[arschles]

We have external storage drivers now, so an external driver could be built, using the Athens pkg/storage packages, that supports deletions. I'm going to close this for now since there hasn't been activity here in a long time. If anyone comes across this issue and has a definitive use case to having Athens support deletions, please feel free to comment here or open up a new issue 😄