Remove Provider.UploadURL

[applejag]

Old and unused field on the provider model.

For GitHub, the API URL is https://api.github.com/, but to upload assets to a release you use https://uploads.github.com. This URL is however provided by the "Create a release" endpoint and should be able to be provided from there instead.

I've not researched the other providers, but I do know that the upload URL field is unused throught Wharf.

Migrations are needed. GORM doesn't removes columns automatically, it only adds them automatically.