For the profile metadata link fields (#323), we should pull the link text out of the parsed mf2 and use it in attachment.name when available instead of hard coding Link. And fall back to title if text isn't available.

Specifically, we'd look up each URL in the parsed mf2's rel-urls, eg:

    "rel-urls": {
        "https://twitter.com/intent/user?screen_name=t": {
            "title": "follow me on Twitter",
            "text": " Twitter @t",
            "rels": [
                "me"
            ]
        },
        "..."