Content categories APIs

[haoliangyu]

The APIs to manage content categories are missing:

https://developers.arcgis.com/rest/users-groups-and-items/assign-category-schema.htm
https://developers.arcgis.com/rest/users-groups-and-items/category-schema.htm
https://developers.arcgis.com/rest/users-groups-and-items/delete-category-schema.htm
https://developers.arcgis.com/rest/users-groups-and-items/update-items-with-content-categories.htm

[tomwayson]

@dbouwman and I noticed that the home app uses https://developers.arcgis.com/rest/users-groups-and-items/update-items-with-content-categories.htm when reviewing #777