Replace no_filter with None

[tavinathanson]

From @arahuja:

We definitely need a way to not filter which no_fitler solves, but for some reason, I'd rather None did that? Does it make sense for filter_fn to have 3 possible args: 'default', a filter fn, or None. Where 'default' goes to the cohort? This way the user never needs to know of anything special, either the default happens or the user specifies a filter function or None? I worry about the discoverability of no_filter and just None being more natural?