microformat mentions can have an implicit url property

[nightpool]

See the first example here: http://microformats.org/wiki/microformats2#hyperlinked_person

Simple hyperlinked person reference
<a class="h-card" href="http://benward.me">Ben Ward</a>

Parsed JSON:

{
  "items": [{ 
    "type": ["h-card"],
    "properties": {
      "name": ["Ben Ward"],
      "url": ["http://benward.me"]
   }
  }]
}

see http://microformats.org/wiki/microformats2-parsing#parsing_for_implied_properties for a more detailed list of implicit property parsing rules.

We might also want to change the HTML mastodon generates at some point, since having an extra span element feels awkward to me.

[Gargron]

There is an extra span for a better underline effect on hover that does not include the at sign, we won’t remove that

[ClearlyClaire]

Approved. But I think we could also skip URLs for hashtags and mentions listed in the ActivityStreams tag attribute.