cleanup xmodule defaults

Fix markdown defaulting logic to be require explicit setting to trigger
simple editor rather than defaulting.
Fix dedent of default xml
Don't override display_name defaults on unit creation
Ensure xml based course problems get the old defaults but new problems
get the new defaults.
Fix tests which assumed the old defaults but didn't use xml for
creating.
Treat about/overview as boilerplate not default about
Sort templates by name (db or get_templates() was doing this before?)
Interesting bug where null as the value of metadata implied reset to
default which screwed up things w/ default markdown