Bug 9677 - Style button on notices and slips page with Bootstrap

The toolbar button on the Notices & Slips page isn't styled with
Bootstrap as other toolbar buttons are. This patch corrects this.

The patch also removes some obsolete CSS.

To test go to Tools -> Notices and Slips. The toolbar button should look
correct and work correctly.

Patch works as expected.
Signed-off-by: Marc Véron <veron@veron.ch>

Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
"yui-menu-button" is not contained in tt files.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>
(cherry picked from commit 36eac129f7fdab89ffd77ba5f3bd3add688deedc)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>