Allow @mentions in template variables

I have cloned this Ticket from CONF-28476 because I think it was closed without ever being solved. I think the importance of this is significant to the usefulness of the product:

• For example: Be able to reproducibly assign people to many tasks in a confluence task or checklist just by filling in a few fields

Merely using the instructional text does not allow for the ability to auto populate an entire template if the mention happens many times in something like tasks.

Also, dates should also be able to be used in template variables.

From old ticket:
It is currently not possible to @mention someone in a template variable. Allowing this could improve uptake of Confluence for new customers, by increasing visibility of Confluence in the business.