UnknownFailure: add "build_name" to filter options
Add "build_name" to filter options of UnknownFailure model to enable filtering issues by build names. It is a useful feature when dealing with sporadic regressions that affect a few builds, which have produced invalid results. Log signatures are not always unique in these cases, so creating a bug for such issues is not easy.
This feature is only added to user query, because it is supposed to be used rarely - no need to add any GUI element for that.