Expand get_dropdown_options()

[Samlant]

Build upon get_dropdown_options() so that adding more carriers is easy and more extensible rather than just creating a list. This will be better as new carriers will only need to be added once, and it will be automatically sorted as a potential redundant carrier or as a normal, single carrier.

Steps to take:

• Revise get_dropdown_options() from a list() into a dict() like {carrier_name:'', potential_redundancy:bool, var:'', text_to_display:''}, and change it to a private method.
• Add method in model.py/Model class to divide potential redundant carriers from single submissions.
• Use Presenter class to get carriers from Model class
• Create a _create_variable() fn to create variable name to be used in checkbox