Handle user edit for all groups with same code.

This refactors the user list/add/edit/delete code so each group is
handled by the same code. That also required renaming forms and views
so they matched the group name (journal_manager -> vesting_manager).

This is kind of a big change (in terms of potentially creating bugs) with nothing user-facing to show for it, but I think it will make the user edit pages a lot easier to update and keep in sync from here on out.