Introduce project settings presenters
Description
We are working on combining settings gear navigation with existing tabbed navigation (see https://gitlab.com/gitlab-org/gitlab-ce/issues/23007).
The first step is to combine members and groups settings pages (https://gitlab.com/gitlab-org/gitlab-ce/issues/25985).
In a related merge request discussion we were thinking about an implementation detail called "settings presenter". More description can be found in comment.
Proposal
Introduce settings presenter which would become objects needed to render settings form. With this approach we can shuffle settings presenters between tabs. This design pattern is meant to decouple settings pages (index/show action) from controllers and would improve testability.