[MERGE][FIX] website_(sale)_slides(_survey): apply various UI fixes and improvements

Purpose of this merge is to provide some fixes and tweaks related to design
of eLearning application. Notably this merge provides fixes for mobile
display, forum integration, certification-specific design and some cleaning
in SCSS and bootstrap customizations. See sub commits for more details about
them.

Linked to task 1958372
Related to PR #32648

Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>