[MIGRATE] website_forum: migration to the new API, cleaning and linting of models and controllers.

Main cleaning
- remove introduction_message, introduced in saas-6 with inboung forum, that was quite
a duplicate of description.
- renamed relevancy_option_first and relevancy_option_second, introduced in saas-4
with inbound forum
- renamed type of post to post_type to avoid confusion with the type keyword
- some cleaning in templates
Merge branch 'master' of https://github.com/odoo/odoo into master-linux64