Replace fitting via CREST

[mkaito]

Currently, exporting a fitting that already exists creates a copy. I'd rather it replace the existing fitting.

[blitzmann]

It is possible to do this in a round about way. Each saved fitting in EVE has a fitting ID. When importing from EVE, we can save this ID to the fit so that when we go to export it again, we can use it to delete the old one and then upload the new one (which should produce a new ID). When uploading a new fit to EVE, we can simply save the created ID for future use.

It's something that I thought of doing when developing the initial CREST support, but it's going to be a fairly big change. This will go in the backlog for now.