Bug 27695: (follow-up) Add specificity to CSS changes

Bug 27695: (follow-up) Add specificity to CSS changes

This patch moves the definition of .alert and .error classes into the
block for .dialog in order to make sure they don't have an effect beyond
the desired scope.

The ".closebtn" section is redundant and has been removed.

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
(cherry picked from commit c42c24fa03709496d0c21b98fe7db228f02d3983)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>