Bug 14783: (follow-up) Move pickup library form into modal

Bug 14783: (follow-up) Move pickup library form into modal

This patch attemps to simplify the list of holds by moving the form for
changing a hold's pickup library into a modal dialog.

This patch also makes minor tweaks to the CSS for the "X" close button
shown in modal in windows. Instances where modal markup used "x" have
been updated to use the Bootstrap 4 standard "×"

To test, apply the patch and rebuild the OPAC
SCSS
(https://wiki.koha-community.org/wiki/Working_with_SCSS_in_the_OPAC_and_staff_client)

- Turn on the OPACAllowUserToChangeBranch system preference.
- Log in to the OPAC as a user with holds.
- In the view of your holds, confirm that each hold which can be
  modified shows a "Change" link.
- Clicking the link should show a modal window with the title of the
  item on hold shown in the header.
- Confirm that changing the pickup location works correctly.

Signed-off-by: Barbara Johnson <barbara.johnson@bedfordtx.gov>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>