[MERGE][IMP] website_slides, website_*: email validation, comment restrictions with karma and search user profile

This merge is improving various points linked to eLearning.
Impacted modules: website_profile, website_forum, website_rating and website_slides

- Move validation email from website_forum to website_profile to use the feature in both Forum and elearning. The purpose is to give first karma points to allow users to comment, vote, ... on slides.
- Add search bar in 'all users' page. To do so, we needed to introduced the `karma_position` field.
- Restrict comment, review and vote based on user's karma in eLearning (minimum karam required to review/vote/comment, like in forum). This feature required to add 3 stored fields, with the benediction of chs !
- Fix channel completion computation and linked karma gain. Also provide some test on karma gain.

See sub commits for more details.

Task-1943788

closes odoo/odoo#31321

Signed-off-by: Jérome Maes (jem) <jem@openerp.com>