Convert BoardsHelper into a presenter

The following discussion from !9983 (merged) should be addressed:

@brodock started a discussion: (+4 comments)

This whole class can probably be converted to at least one Presenter, as we are essentially using it to deal with data for the view, and not to generate/augment the html content.

You can read a little bit more about it here: https://gitlab.com/gitlab-org/gitlab-ee/blob/master/app/presenters/README.md

By converting it to a presenter, we can make it much easier to test, as we don't depend on specific instance variables in a controller, as everything required is passed to the object.

/cc @smcgivern - this is a technical debt raised (not introduced) in the discussion above, I set 11.11 to not forget about it, but please assign appropriate milestone.

Edited Apr 02, 2019 by Jan Provaznik