Orphan gender filter

Jira story: https://osraav.atlassian.net/browse/OSRA-204

- Added gender filter to Orphan index view similar to the one we have for Sponsors
- DRYed gender string constants and put them in yaml config file
- DRYed custom filter feature

OSRA-204#Code-Review
As long as the chore is created so that we don't lose track of this, leaving it in settings is fine by me.