working on new rights.

* attachments can have authors, now.
* officers can create pages in their domains
* they can edit their own pages
* they can upload attachments
* they can edit and destroy their attachments

trello: https://trello.com/c/1UqSsBHx/681-neue-rechte
groups#show: making authorization more efficient.

Using the cancan `accessible_by` scope is faster than looping over all
objects and calling `can?` separately. But, in order to make it work,
we could not use a block for `can :read, User` anymore.
using rspec-rerun to rerun failing capybara tests, especially on travis.

https://github.com/dblock/rspec-rerun