Fix XSS in /artist_versions.

1) Put `<script>alert("xss 1")</script>` in the Other Names field in an
   artist entry.
2) Put `<script>alert("xss 2")</script>` in the URLs field.
3) Trick someone into the viewing the history page for that artist.
Merge pull request #2275 from evazion/xss/artist-history

Fix XSS in /artist_versions.