Add WYSIWYG Content Builder

[YaWK]

Improve WYSIWYG View with a content builder.
(Bootstrap components that can be dragged from a sidebar)

The new editor will feature inline-editing. This will improve usability and make YaWK much more accessable to less experienced users. Pros out there, don't worry: the source editor (codemirror) will stay, for sure. You will always have the option to switch fluid between those two editor modes. At settings/backend settings you can select in which mode the editor should start on default.

This is a huge feature and it will take some time to finish - it is also a preparation for the implementation of Chat-GPT (See this Milestone)