Netlify CMS with bitbucket server

**Is your feature request related to a problem? Please describe.**
Yes. I would like to use Netlify CMS with Bitbucket server. I first tried to set it up with OAuth but ran into issues there, mainly because I didn't want to use Netlify Auth. The other approach that would probably be even better for me is there would be able to be support to just use the bitbucket cookies which are supported out of the box by bitbucket and won't require any changes to the bitbucket application itself.

**Describe the solution you'd like**
Support to login to bitbucket and then use its cookies, rather than OAuth.

**Describe alternatives you've considered**
See above