Entity-ise msgids in po output.

Before:

```po
msgid "affected"
msgstr "\"><img src=x onerror=alert(1)>"

msgid "affectionate"
msgstr "\"><img src=x onerror=alert(1)>"

msgid "afflicted"
msgstr "\"><img src=x onerror=alert(1)>"
```

After:

```po
msgid "affected"
msgstr "&quot;&gt;&lt;img src=x onerror=alert(1)&gt;"

msgid "affectionate"
msgstr "&quot;&gt;&lt;img src=x onerror=alert(1)&gt;"

msgid "afflicted"
msgstr "&quot;&gt;&lt;img src=x onerror=alert(1)&gt;"
```

Needs integration testing with publisher with live data... has this been done in the past? @yegg @sdougbrown @zekiel @nilnilnil