Prevent upload to unauthenticated users

Includes:
* redirecting unauthentiated users to sign in page
* upload validation tests now use splinter

@alexmuller
@maxfliri
Decrease splinter timeout to 3 seconds

@alexmuller
@maxfliri
Allow user access to upload based on permissions

@alexmuller
@maxfliri
Refactor tests for readability

@maxfliri, @alexmuller
Permissions checking extracted to class

@alexmuller, @maxfliri
Add unit tests for Permissions class

@maxfliri, @alexmuller
Method renamed to a more sensible name

@alexmuller, @maxfliri
Fix user name in page top bar

@alexmuller, @maxfliri
Merge pull request #92 from alphagov/signonotron2-bucket-credentials

Signonotron2 bucket credentials