EZS-14 - Build "Create Content" module in the sidebar of PlatformUI

Created a sidebar module to allow a user to create a new content.

https://jira.ez.no/browse/EZS-14

**THINGS DONE**
* [x] Fetching content types from the system,
* [x] Filtering a list of content types with search query and selecting content type group.
* [x] Completed the code documentation,
* [x] Prepared JS unit tests,
* [x] Improved the visuals,
* [x] Create content view of PlatformUI

why are you implementing a custom AJAX controller/action, the REST API already has the necessary resources to get the content types groups and the content types and the REST client has a ContentTypeService with the methods `loadContentTypes` and `loadContentTypeGroups`