Add and configure Netlify CMS

[renestalder]

Add the Netlify CMS, served under the '/editor' URL path.

This is a POC to introduce an online editor for #198.

The CMS can be opened on the /editor path (locally only currently).

Tasks (MVP):

• [x] Setup Netlify CMS foundation
• [x] Configure open authoring: Allows non-members of the repository to contribute content via forks through the CMS, which then creates a pull request.
• [x] Configure editor workflow: Creates change requests via pull requests.
• [ ] Configure preview links
• [ ] Add gulp handler to manage the CMS environments via Gulp: When developing locally or showing the result of the work on a preview app, we want that the CMS correctly adapts the configuration to this git environment and branch. Without this, the CMS would always edit on the live environment instead of the local or the preview environment.
• [ ] Find a solution for automatic update of the "changed" property value
• [ ] Find a solution to exclude the example folders: Currently, the example folders are shown in the CMS, yet those examples don't really fit the editor format e.g. nothing is shown when the examples are expanded. Exclude them for the moment.
• [ ] Find a solution to deal with example includes: This probably can only happen via custom widgets.
• [ ] Find a solution for media management: Currently, media is configured to handle the current structure of sub-folders, but those media files don't seem to appear in the global media library yet.

Extras:

• Add an edit link to every page
• Allow setting co-authors e.g. to include this information in the commit message
• Accurate editor previews (currently the editor preview is deactivated)