returning excluded count of failures

This update adds a count of unclassified failures that are within the repo's default exclusion profile.

The ``unclassified_failure_count`` endpoint now returns something like this:

    {
        "count": 99,
        "count_excluded": 28,
        "repository": "mozilla-inbound"
    }

Where ``count`` is the total count of unclassified failures in the last week.  and ``count_excluded`` is the count of unclassified failures in this repo that are excluded by the default exclusion profile for the same time period.