Fix LMS-530, reflected XSS

Limit the page and chapter numbers to digits, to keep malicious URL
components from being inserted onto the page.
Tests and CHANGELOG for LMS-530
Adapt the tests to the new way of making URLs.
Add tests for image books, and more tests for the other kinds too.
Merge pull request #323 from edx/ned/fix-lms-530

Fix LMS-530: Reflected XSS in chapter and page numbers