Burn-down chart: Show subfeatures list (if any) in tooltip popup.

In features-info.json (on the gh-pages branch of the main PSM repository), some of the high-level feature have a non-empty subFeatures list. For example:

    "psm-feature-040": {
        "feature_id": "psm-feature-040",
        "description": "Add new fields for specific requirements relating to bordering-state providers, care settings, help, etc.",
        "subfeatures": [
            "Detect bordering-state providers",
            "Ask specific questions of bordering-state providers",
            "Make state borders and border regions configurable",
            "Screen provider location for in-state or bordering-state",
            "Limit state Medicaid IDs to not conflict with NPIs",
            "Research \"care settings\" in various states",
            "Add care settings UI",
            "Add start/end dates for provider data"
        ],

      ...}

When sub-features are present, we should show them in the tooltip popup. At least, that's what I propose -- see discussion started in Zulip chat for followup.

SolutionGuidance / psm-dashboard

Burn-down chart: Show subfeatures list (if any) in tooltip popup. #40