graphqlbackend: treat yes/true and no/false values the same

Created by: keegancsmith

This commit updates the yesNoOnly parser to not return different values for true and false. Instead it will return yes or no respectiveley. This simplifies read sites since we don't need to check for true or yes, etc.

This also highlighted a weird bit of logic. Previously if you did "fork:yes" we would include forks. However, if you did "fork:true" we only searched forks. I am unsure if this is intentional behaviour, but it is surprising to me. Rather if a user only wants to search forks I'd expect them to search "fork:only".