feat: return full GitHub ref for single-repo keyboards

[mcdurdin]

Currently the keyboard API always returns the short form of the GitHub ref, e.g. release/sil/sil_ipa in the sourcePath prop. However, as we now support remote repositories, we should update the sourcePath prop to allow for arbitrary GitHub repos, paths, and refs.

This will require changes in a number of places:

• keyman.com
• api.keyman.com
• kmc-keyboard-info
• kmc-model-info
• kmc-package, potentially?
• kmc-copy (getKeyboardSourceFromKeymanCloud)
• schemas, database, potentially