Don't require repeating `'split'` when adding ckeditor plugins

[abea]

The problem to solve

When adding ckeditor plugins, you need to include 'split' in the extraPlugins array even though it's included in core already.

Proposed solution

Apostrophe core should concat the project-level plugins with ['split'] so developers don't need to repeat that core plugin.

Alternatives

The main alternative would be telling people to include split in their project-level configuration.