Removal of feature category

The feature category exists to provide a place to discuss about new feature. This was because on the former bug tracker discussion was difficult to follow as they were on a single thread and the editor was not very rich.
But now on Heptapod we can have threads in issue comment and we editor is as rich as the discourse one.

So I think we could just close the Feature - Tryton Discussion category and just request to open an issue report with the label feature.
This way we will get the automatic closing and internal reference features to follow the progress of the feature. And also it will allow to clearly reject a feature request.

For the sub-categories, I propose to move Ideas - Tryton Discussion as a top level category.
Draft Howto - Tryton Discussion will be removed by Readthedocs is going to drop mercurial support - #5 by ced.
And Sponsoring - Tryton Discussion could go under Organisation - Tryton Discussion.