Add comparison module

Roughly extends GroupedTimeseries but has a bespoke collection which makes multiple requests for datasets with different filters depending on the comparison property of the config.
Do not default filter_by parameter in timeshift collection

Leave the parameter undefined if it is undefined.
Allow for pinning the start date of a grouped_timeshift collection
Refactor grouped timeshift collection parse

Use date functions to fill missing data to the desired duration instead of assuming that the first dataset returned contains the correct and sufficient data.
Allow custom classes on grouped timeseries/shift graph components
Allow child collections to overwrite the parent options

To allow comparison collections to extend the parent's filterby property to create comparisons
Apply custom line classes to selection indicators
Remove modules made obselete by new comparison module
Merge pull request #340 from alphagov/feature/student-loans-comparison-module

Feature/student loans comparison module