When we first implemented the datagrids, it was the case that assigning

a list value into a TextField would serialize it as a comma-separated

list of the values in the list. This is no longer the case, and setting

the datagrid columns would end up saving as a list repr, which wasn't

parsable when loading the value back out.
This change fixes it so we explicitly join the datagrid columns when

saving them back to the profile, and adds some unit tests to make sure

that saving the columns and sort order work correctly.

Ran unit tests.
Enabled, disabled, and moved some columns around and saw that it got

  properly saved to the profile.