Enable Category Grouping for Authors

[ojopaul]

We should introduce Author Category at Author level such that:
Each Author can be assign to specific Author Category when editing/adding new author.
When adding Author to new posts, they'll be added to their category (and editor can simply drag to change their category for any post or individual post)
We'll be able to add Author Category filter to Author List so an admin can lists users belonging to a specific Category without post ID

Our current author category post limitation makes it impossible to list users belonging to a category as category is post dependent

[stevejburge]

@ojopaul Would it make sense to move this to the next release? We're already changing a lot in this release