Prohibit @mentioning deactivated users

Hi team! 

I wanted to suggest a functionality around deactivated users; they should not be able to be @mentioned.

We noticed it today that although there is a red Deactivated label when you start @mentioning someone, they can still be tagged. There is no warning when trying to mention them and you can fully send a comment tagging them. 

I think it would be beneficial if:

1. Deactivated users still show up in the dropdown, but are greyed-out and are not selectable when trying to @mention. This way, no communication is lost as some people don't realized the Deactivated label or what it even implies.
2. We could add a customized note that can be seen from these types of screens, similar to how in Outlook you can put an OOO message (i.e. "XYZ is away and will be back on MM/DD. In the meantime, please direct your comments to ABC.") This would allow us to clearly help users know who to tag or redirect their comment to instead.

We have a number of users who are deactivated because they are on parental leave or extended holiday. Allowing users to tag them could result in tasks becoming overdue, unanswered comments that could span weeks/months, and other general complications that can stem from this. Making it more clear to others that someone is no longer active in the platform by blocking them from mentioning them would almost force them to ask around and see who should be contacted instead.