Merge the "plugin boilerplate" branch containing WP VIP fixes and other things

[benlk]

I'm creating this PR to track stuff that started happening in the plugin-boilerplate branch that was never merged with master. It's a bunch of things. Before this pull request is merged, we need to:

• [ ] figure out what changes
• [ ] figure out what issues this plugin addresses

Changes

Questions

Post-merge actions

• [ ] bump version number
• [ ] release

[benlk]

This:

• ❌ adds a lot of boilerplate files
• ❓ deletes the docs?
• 😬 refactors the plugin
• 😕 adds tests for widgets and settings, which are dependent upon the refactoring of the plugin
• ❓ adds Gulp
• ❌ adds SCSS version of the plugin's CSS

and some other stuff.

Would needs a lot of cleanup before merging.