Changed how answers from same page are treated

Answers belonging to **different** questions which belong to the same page are now properly ignored when suggesting better/worse results. Previously, _all_ answers were removed from the suggestion, even if there was just one question on a page, but with multiple answers.
Added name field to Page model

Page model is now identifiable by a name field, so that adding pages to a test will be clearer.
Implemented basic admin management

Added models to admin:
- add/edit tests together with associated pages and results;
- add/edit questions together with associated answers;
- links between questions and pages are made from the questions add/edit page.