Bug 27543: Tooltip on opac-messaging.pl obscured by table headers

Bug 27543: Tooltip on opac-messaging.pl obscured by table headers

This patch corrects which Bootstrap assets are compiled with the OPAC
CSS -- The file for Bootstrap tooltips should be included.

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).

1)  Log into the OPAC.
2)  Go to 'your messaging' (opac-messaging.pl).
3)  Hover over the Digests only tooltip.
4)  Note that the tooltip looks correct.

Signed-off-by: Sally <sally.healey@cheshiresharedservices.gov.uk>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>