Bug 27814: Improve responsive behavior of the user page in the OPAC

Bug 27814: Improve responsive behavior of the user page in the OPAC

This patch makes minor changes to OPAC CSS in order to improve the way
the logged-in user's "your account" page works at narrower browser
widths.

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

 - Log in to the OPAC as a user who has multiple checkouts.
 - Test the page at various browser widths, from > 1200 pixels wide to <
   300 pixels wide. Your browser's built-in responsive design mode,
   found in developer tools, can make these measurements easier.
 - At "phone-size" width the tabs ("Checked out," "Overdue," etc) should
   start displaying full-width.
 - The DataTable controls at the top of the checkouts table should
    adapt well as the browser width changes.
 - At narrower widths the tables on this page should display much better
   than they did before the patch: They should expand to fit the width
   of the page.

Edit: Tweaked the display property of the table search field at narrower
browser widths; Converted iCal download link to button to match other
elements in the toolbar.

Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>