Before this commit, it was not possible to center a button using the
editor option 'align center'.

It is because, using document.execCommand(), the Chrome browser doesnt
want apply 'text-align: center' on nodes with centered children. And
the buttons have the boostrap class '.btn' which adds text-align:
center.

So the hack here is disabling the text-align property from the '.btn'
class during the execution of document.execCommand().

task-2423935

closes odoo/odoo#64461

Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>