Bug 22399: Improve responsive behavior of the basic marc editor

Bug 22399: Improve responsive behavior of the basic marc editor

This patch makes markup and style changes to the basic MARC editor in
order to improve the page's handling of smaller browser widths. It adds
flexbox styling to elements of the form to help accomplish this.

To test, apply the patch and load a record for editing in the basic MARC
editor.

- Adjust your browser width and confirm that the form adjusts well
  to various widths.
- Test that the markup changes haven't broken the JavaScript-driven
  features in the editor:
  - Clone tags
  - Delete tags
  - Expand and collapse tags
  - Clone subfields
  - Delete subfields
  - Re-order subfields
  - Cataloging plugins:
    - Focus-driven plugins like on tag 000
    - Popup plugins like 008 or authority-linked tags
    - Select2-styled dropdowns (e.g. 942$c Item type).
- Test the add/edit items form to confirm that it is still styled as it
  was before.

Edit: The Select2 plugin JavaScript file now accepts a variable to set
the "width" configuration option. It still defaults to "element."

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