Bug 21531: Subscription "New fields" button should read "New field"

Bug 21531: Subscription "New fields" button should read "New field"

This patch changes the label of the "New fields" button on the
subscription add fields page to read "New field," making it consistent
with similar buttons in the staff client.

Also changed:

- Add missing id to <select> to make label clickable.
- Remove custom DataTables configuration in favor of defaults.

To test, apply the patch and go to Serials -> Add subscription fields.

- The button in the toolbar should read "New field."
- The DataTables' functionality of the table of existing custom fields
  should be correct.
- Click the "New field" button.
  - Cliking the "Authorised value category" label should put the focus
    on the dropdown.

Signed-off-by: Claire Gravely <claire.gravely@bsz-bw.de>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>