REDO [DEV-2155] Add date_joined field to AskbotUser.

This is a redo of the previously-merged DEV-2155 work.  We decided not to ship it until after the holidays, but we did want to ship DEV-2354, which was merged on top of it.  So, I removed the DEV-2155 work and re-added DEV-2354.

This request will re-add the DEV-2155 work so that it can be deployed after the holidays.