Bug 31861: Table controls on checkouts table are buttons

Bug 31861: Table controls on checkouts table are buttons

This patch updates the style of DataTables buttons (columns, export, and
configure), in cases where the buttons are the only control being
displayed (no page controls, filter, etc).

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

- Go to Circulation and check an item out to a patron.
- If necessary, click the "Show checkouts" button to reveal the table of
  checkouts.
- The "Columns," "Export," and "Configure" controls should look the same
  as they do in other situations, with no button-style border.
- Go to Circulation -> Check in and check in the item. The table of
  checked-in items should show the same style controls.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>