Remove duplication in chart forms code

[David Platten]

forms.py contains duplication between the XXChartOptionsForm and XXChartOptionsDisplayFormmethods for each modality. One of these is used to display chart options on the modality filtered page, the other is to display the options on the over-arching chart options page. The pairs of methods could probably be merged into one using nested / inner classes: https://www.datacamp.com/community/tutorials/inner-classes-python

This would reduce duplication and make coding errors less likely.