Release v1.15

I have just release v1.15 of the Dlayer demo; I had more features planned, but after a refactoring task increased in scale I opted to stop adding new features and save them for the next release.

The full changelog for v1.15 is below:

* Enabled the rich text content item tool. [Feature]
* Added a blog post content item [Feature]
* Added styling, typography and delete sub tools to rich text and blog post content items. [Feature]
* Original text content item renamed to ‘plain text’.
* Added PHP Quill HTML renderer, used by the tools which have rich content areas.
* Tool links inside an empty column dynamic.
* Refactoring.
* Minor fixes and improvements.