244 filters

https://osraav.atlassian.net/browse/OSRA-244

- changed filters for Orphan model
- changed filters for Sponsor model
- in order to filter sponsors by the number of sponsorsips, created an active_sponsorship_count column in the Sponsor model, which changes the value every time a sponsorship gets created or inavtivated

+ rspec tests

(paired with Cristian and Brian on this task)