Do not redirect to editor if parameter does not equal edit.

Closes #2619
- If edit parameter is 'edit' redirect to editor.
- If edit parameter is anything other then undefined redirect to 404.
- Create edit post tests.
- Test redirect without trailing slash.
- Test redirect to editor.
- Test redirect to 404.
Merge pull request #2624 from shindakun/editbug2619

Do not redirect to editor if parameter does not equal edit.