Automatically fill in 'publish.location' in config when absent

[fsoikin]

Description of the change

Fixes #1060

• When publish.location is missing in the config, Spago will try to figure it out from the origin remote.
• If figured out successfully, the location is written back to spago.yaml, but the publish operation is still aborted, because now spago.yaml needs to be committed to Git.
• If there is no remote named origin, Spago gives up.
• If the origin remote has a GitHub URL, Spago uses the owner+repo format, otherwise - the sole URL format.

Checklist:

• Added the change to the "Unreleased" section of the changelog
• [ ] Added some example of the new feature to the README
• Added a test for the contribution (if applicable)

[f-f]

Thanks!