We should consolidate on a good way to get a display name for fields.

Both `Audit` and `FieldPermission` have essentially completely identical methods (`field_display_name` and `display_field_name` respectively), names are defined in HTML when used with the `field` templatetag, and that falls back to the `help_text` in the field definition if a label isn't supplied.

IMHO the field's `help_text` is preferable to the process used in `display_field_name` since it allows the display name to be unrelated the the Python field name, but this is probably a matter of some debate.