multiple valid values for discrete filters

Created by: attfarhan

In our frontend parser and in other places in the webapp, we match "yes" or "no" exactly when identifying values passed to discrete value filters (case, archived, fork, etc.). In fact, there are multiple values to denote yes and no values: https://sourcegraph.com/github.com/sourcegraph/sourcegraph@fa/fix-case-yes/-/blob/cmd/frontend/graphqlbackend/yesnoonly.go#L16:21