Bug 35426: Improve layout of bookings modal form

Bug 35426: Improve layout of bookings modal form

This patch changes the class on the bookings modal form's fieldset in
order to make the form work better in the small space offered by the
modal.

Also changed: The hint for the patron search field has been moved below
the field for consistency.

To test, apply the patch and search for a title in the staff interface.

- View the detail page.
- If you don't see a "Place booking" button in the toolbar,
  click "Items" in the sidebar menu.
  - Locate the "Priority" heading, and change the "Bookable" setting to
    "Yes."
- Click the "PLace booking" button in the toolbar.
  - The bookings modal form should appear, and the form layout should be
    improved.

Signed-off-by: Barbara Johnson <barbara.johnson@bedfordtx.gov>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>