Forbid "." and ".." in slugs
Summary: Fixes T4614. These don't do anything bad or dangerous, but generate unusable pages.
Test Plan:
- Added and executed unit tests.
- Tried to create pages like /../, /begin/../end/, etc.
Reviewers: btrahan, chad
Reviewed By: chad
Subscribers: aran, epriestley
Maniphest Tasks: T4614
Differential Revision: https://secure.phabricator.com/D8535