Add profiling decorator

...and repackage profiling middleware.
Default to having debug toolbar disabled

Makes sandbox MUCH faster.
Use cached version of categories on listing view
Tests: Check new password works after changing it.

The profile test can_change_password checked if the password change
form can be submitted succesfully, but did not verify that Django's
authentication system accepts the new password.
Merge pull request #580 from tangentlabs/issue/580/docs

Docs: Contributing guidelines and legacy docs
Fix mapping quantities to selected lines
Use default filter to avoid None in templates
Adjust settings docs for slugify function
Merge pull request #622 into releases/0.5