Add a setting switch for js18n

We are getting 404 requests on the jsi18n url because the plumbing is
setup, but all of our translation is currently handled by server-side
code. This switch allows us to turn it off until needed.
Allow single quote characters in UDF names
Better validations and helper methods for UDFs
Catch ValidationError and return 400 with json
Merge pull request #288 from jwalgran/topic/jsi18n-switch

Add a setting switch for js18n
Clean up pep8 lint in opentreemap/urls.py
Merge pull request #289 from ahinz/topic/udf-editing

Prep for editing udfs