Enforce varname formatting for CODEMIRROR_FIELD_INIT_JS setting so it
should always be a valid Javascript variable name.
Concretely varname is slugified and dashes are replaced by underscores so
a given varname like foo codemérror-0 is turned to foo_codemerror_0.
This should be a safe change and resolve issues with usages inside a formset.
Thanks to @abumalick for report.
Also added support with Django 2.2 and removed notices about major changes
from 2.0.0 version.