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
@motasem-salem We had a discussion in last night's scrum about using config/settings.yml this way. Our take on it was that, as the name implies, this folder is for configuring the app's settings, not for storing common code. A more natural place to keep common model code would be app/models/concerns - as yaml or modules. For the importer, which lives in /lib, we could add a e.g. /config subfolder for the yaml settings file. How does that sound to you?