Bug 26512: Display issue with buttons for OPAC checkout note

Bug 26512: Display issue with buttons for OPAC checkout note

This patch adds the correct Bootstrap 4 class to the buttons for
submitting checkout notes in the OPAC.

The markup has been updated to use <button> instead of <a> because of a
style conflict with jQueryUI's CSS.

To test, apply the patch and enable the AllowCheckoutNotes system
preference.

- Log in to the OPAC as a patron with checkouts.
- On the "Your summary" page, enter some text into a field in the
  "Notes" column of the checkouts table.
- Upon typing in the field a button should appear, "Submit note."
- The button should be styled as a green button.
- Submit the note.
- Make a change to the text in the field. A button should appear,
  "Submit changes." It should also be styled correctly.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>