(bug 62210) remove EntityContent from SpecialPages.

This includes some refactoring, in particular pulling up functionality
from SpecialModifyŦerm to SpecialModifyEntity, and passing the
entity as a paramter instead of relying on a member variable.

Change-Id: Ia92e909b4e4585b3c42dd6ed1e35d9e7d4b2f97a
Merge "(bug 62210) remove EntityContent from SpecialPages."