Bug 28537: (follow-up) Correct and improve OverDrive checkout modal

Bug 28537: (follow-up) Correct and improve OverDrive checkout modal

This patch corrects the markup of the modal which is displayed when the
user clicks the "Download as" button in their list of OverDrive
checkouts. The modal markup had not been updated in the conversion to
Bootstrap 4.

I have also modified the markup generated when listing the download
format options so that they use some standard Bootstrap 4 classes,
improving the layout a little.

To test you must have valid credentials entered in system preferences
for the OverDrive API. You should test using a patron who has holds and
current checkouts in OverDrive.

Apply the patch and log in to the OPAC.

- On the "Your summary" page, open the "OverDrive Account" tab.
  - Log in to your OverDrive account.
  - In the list of current checkouts, click the "Download as" button for
    one of the titles.
    - A modal window should display with a list of radio buttons for
      each available format.
      - Confirm that the format text acts as the label for the radio
        button.
      - Confirm that closing the modal without making a selection works
        correctly.
      - Confirm that making a selection and submitting the form closes
        the modal and submits your choice.

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>