Fix news and announcements icons on the frontpage

Feature/cale/cms save validation

Load modules from the mongo modulestore as errordescriptors if creating a regular descriptor fails, but leave the location the same

Make imports into the CMS ignore broken modules, while still loading bad xml as error modules in the LMS

Make ErrorDescriptor continue to use the interface for __init__ that XModuleDescriptor uses, while providing a helper function for constructing from a different set of arguments

Move common code for updating modules in mongo to a single helper function that also handles creating the item if it doesn't already exist

Elseview press release

Add links from the editing page for the cms to the corresponding page in...

Cdodge/meta data cms editing

Define 'system metadata' which should not be editable by the end user. When posting back metadata edits, we need to fetch a copy of the existing metadata and apply the changes.

Quick fix for scores within a homework/lab showing up in reverse order.