Remove redundant dataset permission

It is not used in admin app, backdrop admin or stagecraft.
Merge pull request #52 from alphagov/remove-dataset-permission

Remove redundant dataset permission
Add in code quality
Merge pull request #54 from alphagov/add-quality

Add in code quality
Add assertion that session doesn't contain element

Add a method assert_session_does_not_contain that asserts that a
KeyError is raised for a given key when accessing the session.

Change the existing assert_session_contains method so that it
is the only thing checking for a KeyError and raising an
AssertionError because of that. This is because otherwise
we would be asserting that an AssertionError is raised, which
is confusing.
Implement dashboard creation form

This commit introduces a form that will POST to Stagecraft to
create a dashboard.

It makes use of the session to store the form if it fails validation,
and then presents it back to the user.
Merge pull request #53 from alphagov/73958524-create-dashboard-form

[#73958524] Create dashboard form
Give the development user dashboard permissions
Add a footer

Adding a footer helps the user figure out when the page has ended.

Also it means that forms don't go right up until the end of the page,
making it harder to submit.
Add module creation to dashboard form