Add Netlify Identity adapter

[Jinksi]

I'm submitting a

Feature request

Description

Related to #77

Add Netlify Identity / Git Gateway adapter.

Netlify Git Gateway handles the Github API requests through it's own endpoint /.netlify/git/github/.

This request is paired with a JWT supplied by netlify-identity-widget

see Git Gateway docs & Netlify Identity docs

[Jinksi]

Hit a roadblock, unfortunately Netlify’s git-gateway doesn’t support the /merges/ endpoint…
Would you consider allowing Simpla to commit changes directly to the target branch rather than merging?

[Jinksi]

This PR (https://github.com/netlify/git-gateway/pull/12) adds the /merges/ endpoint