Bug 12716: Allow the import patrons form have drop-downs and datepickers

To reproduce:
- Go to Tools > Import patrons
- Notice branchcode and categorycode don't have dropdown menus with
  valid options.
- Notice dateofbirth, dateenrolled and dateexpiry don't have date-pickers
- Notice each input field is labeled with the column name instead of a
  proper description.

To test:
- Apply the patch, go to Tools > Import patrons
- Notice branchcode and categorycode have dropdown menus with valid options,
  and by default the empty value is selected.
- Notice that if you go into the dateofbirth, dateenrolled and dateexpiry
  fields, a nice date picker widget appears.
- Notice each field is labeled with the field description, and that the
  column name is conveniently shown at the right of the input field.

Important: test switching the ExtendedPatronAttributes syspref, and verify that
if enabled, the patron_attributes field appears, and also the checkbox selector
controlling the extended patron attributes import behaviour shows too.

Bonus points: Verify that on a different language, the descriptions get translated,
and the column names show correctly

Regards
To+