HTML should be stripped from metadata

If I specify a post's title as the following:

```
title: I read _Green Eggs and Ham_
```

Then the resulting HTML includes a metadata line like this:

```
<meta property="og:title" content="I read <em>Green Eggs and Ham</em>"/>
```

Those HTML `<em>` tags proceed to render literally in many (most? all?) preview cards, and it looks terrible. We should just strip them.