Add support for item category management in Outlook on the web (compose mode)

Unfortunately, the API for managing item categories in compose mode in Outlook on the web is currently not supported: 

In Outlook on the web, you can't use the API to manage categories applied to a message in Compose mode.

https://learn.microsoft.com/en-us/javascript/api/outlook/office.categories?view=outlook-js-1.8#outlook-office-categories-getasync-member(2)

The lack of this API results in a bad experience for our users, since it works in Outlook for Mac und Windows.