set default editor choices for release/2.7

classlist editor 2, homeworkset editor 2, and PGProblemEditors 2 and 3 (need both because editor 3 is iffy with older themes
Provide graceful fall back if theme is not available.

This is a bit of a hack since it hardwires math4 theme as the default theme that is always available.  A future iteration should make this defaultDefaultTheme configurable in defaults.config. (We should also clean up the variable names. :-) )
Merge pull request #74 from mgage/release/2.7

Add graceful failure if theme is not available.