Add position filter to calendar (e.g. for ability to filter out bills that are being tracked but don't have a position)

[keirstan-techequity]

Request from Megan:
it would be good to be able to filter the cal not just by dashboard but also by position. There are bills we have on the dashboard that we are keeping track of but won’t take a position on, when it comes to calendar feature we mostly care about letter deadlines for bills we have a position on v. everything we are tracking.