Merge profile "About" into main profile and simplify some custom field stuff

Summary:
Ref T1703. Drive "user since" with a custom field and make the other fields render into a property list.

Users can make their profiles a little more personal/obnoxious now.

Also delete a bunch of code.

Test Plan: {F49415}

Reviewers: chad, btrahan

Reviewed By: chad

CC: aran

Maniphest Tasks: T1703

Differential Revision: https://secure.phabricator.com/D6401