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.
Move css files out of static directory and compile sass files to css
Concatenate vendor css
Minify CSS files and use minifed version when not in Django DEBUG mode

We're using node-sass, which unlike normal sass does not yet have
built-in compression.  When that is added, we should switch to have sass
compress main.css
Allow single quote characters in UDF names
Better validations and helper methods for UDFs
Catch ValidationError and return 400 with json
Merge pull request #286 from maurizi/topic/compileSass

Compile sass
Guard against null tree_ids

Since None is a 'valid' tree id for this view function, we need to
handle it first
Add in example images for testing
allow testing server to override
Merge pull request #290 from ahinz/topic/flags-for-selenium

allow testing server to override
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
Support editing 'choice' udfs. Fixes #285.
Support editing 'choice' udfs.  #285.
Merge pull request #291 from ahinz/fix/285/edit-choice-fields

Support editing 'choice' udfs. Fixes #285.
Add URL test for creating a tree
Remove unused argument to buildSearch()