Feature/victor/xml verify

* may be temporary if we move errors into the items themselves.

Only import from github when changes are made, rather than exporting. Th...

Conflicts:
	lms/urls.py

* move html_checker.py
* move stringify.py
* move tests into module
* remove duplicate progress.py
* fix module imports

* cleaned up error module:
  - only one template
  - save error message in xml and reload
* better display of problem definition and metadata to staff
* save error messages as string, not exception objects.

Feature/victor/metadata inherit

Only import from github when changes are made, rather than exporting. This prevents bugs in the cms from preventing changes via git

* problem was on import to json--got all the metadata,
  but didn't preserve the _inherited_metadata
* added own_metadata property, use it instead

* also make a placeholder roundtrip export test

* make logging work properly in cms, dev
* fix a comment

Strip down sequence nav, start to use ems for font size instead of pixels and started to strip down the info page

Added more styles for the info page and added headers for the updates and handouts so they will be consistant for every class

Jarv/prod requirements

add back-compat code to customtag

* apparently 6002x is using the impl-as-child structure already.

Feature/victor/xml verify

* Log formatted traceback string instead of exc_info tuple itself
* display as a list

* call error tracker when needed
* remove duplicate logging--just add info and re-raise
* xml modulestore uses error tracker to capture load errors
* add unstyled list of import errors to courseware homepage!

* simplify logic--tracker just tracks errors.  Trackers should not raise,
      and are not be responsible for logging.
* adapted code to use trackers.
* Started cleanup of error handling code:
  - if need to add info and re-raise, just do that.  No logging.
  - if working around a problem, log and track as needed.

* get rid of dump to json
* formatting

* so they get the staff view of problems
* does NOT actually set User.is_staff

* change references and tests
* add staff/non-staff display
* added is_staff to ModuleSystem