CMS-3924 Make ContentTreeGrid support partial loading of all children

Added new classes, such as `ContentMetadata` and `ContentResponse` to support new response format.
Added `TreeNode.maxChildren` parameter, so the display format methods can have full info.
Multiple changes to the format methods, fetchers and other elements, that work with the Content. Added some checks for the empty node with `ContentSummary` equals `null`.
Special css for the ''"x nodes no loaded"'' empty node in class `.children-to-load`.
Placed additional children load into the DblClick event handler.
Added static `MAX_FETCH_SIZE` parameter to the `ContentTreeGrid`. It is equals 5 for now. The parameter was made `static` due to the early root fetching, that is performed in the constructor, as well as unique nature of the `ContextTreeGrid`.
Full changes can be seen in the separate commits.