Added an extra filter argument to the query.

Currently when using this function to get all the plugins for a page,
this returns ALL plugins from ALL placeholders. Which does not take in
consideration old placeholders that might have been removed from the template.
As a quick fix I make sure to get only plugins whose placeholder slot is in
the current template.
Added tests for get_plugins_for_page.
Merge branch 'develop' into patch-1

Conflicts:
	cms/tests/plugins.py