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] Written JS unit tests,
* [x] Improved the visuals.

**THINGS TO BE DONE**
* [ ] Create content view of PlatformUI

CS: Space after and before parenthesis.

Nitpick: avoid temporary variables like `$contentTypeGroups`:

```php
foreach ( $this->contentTypeService->loadContentTypeGroups() as $group )
{
    // ...
}
```