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