[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
[IMP]Sales:Improved some usability issues.

[FIX]improve domain and override create method in product attribute value.

[IMP]Added placehoder in comment field,seted create and delete false in tree view of attribute value

[IMP]Added form view for attribute values and bind with action

[REM]Removed unused code as per spec changed

[REM]Removed unused code for product attribute values and rechange placeholder for purchase.