Bug: missing config.yml validations

[erezrokah]

Going to list here missing config.yml validations that I encounter. Feel free to add more.

• [x] Unique fields names #3804
• [x] Widgets specific field configuration #3841
• [x] Unique file names (https://github.com/netlify/netlify-cms/issues/1035) (https://github.com/netlify/netlify-cms/pull/3913)
• [x] Unique collection names (https://github.com/netlify/netlify-cms/issues/1035) (https://github.com/netlify/netlify-cms/pull/3913)
• [ ] Backends configuration (missing repo, app_id for implicit auth, etc.) Related #1693

[erezrokah]

Re-opening as I added backends validation to the list. We can use the same approach as https://github.com/netlify/netlify-cms/pull/3841