Fix XSS issue with the unordered_list filter

When using the unordered_list filter in a Horizon table (as opposed to
a template directly), autoescaping is not set by default and the input
wasn't sanitised.

Closes-Bug: #1349491
Change-Id: Id82eefe48ccb17a158751ec65d24f3ac779380ec
Merge "Fix XSS issue with the unordered_list filter"