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.
2.4 upgrade typos + formatting
updated page_url section to explain how references to incorrect reverse IDs are handled
fixes a uni language problem
Merge remote-tracking branch 'origin/pr/1639' into develop
Fixed PageAdmin.change_view failing when LocaleMiddleware is not
installed

Thanks to @czpython for reporting this bug and helping fixinging it
Merge remote-tracking branch 'origin/pr/1621' into develop
Fixed Issue #1302
Added tests for get_plugins_for_page.
Merge branch 'develop' into patch-1

Conflicts:
	cms/tests/plugins.py