MixevalQuestion validate multiplier is numeric.

Also helper function for internationalization cause I don't want to keep
typing true for every single string.
Fix validation error not showing in mixeval add.

This was due to the multiple save calls, I guess each save call
overrides the validation error. Fix was to perform validation on the
entire form before actually saving anything.
Add tests for Mixeval add.

These are basic tests taht just makes sure data is making it into the
database correctly.