New template helper: escapeContent

The content field in our api responses varies depending on what
was requested, it can be html, wikitext, or plaintext. Previously
we were using {{html content}} in some places, but since this
could potentially be wikitext or plaintext that opens up XSS vectors.

This patch replaces
    {{html content}}
with
    {{escapeContent contentFormat content}}

Which looks at the reported contentFormat and escapes appropriately

Change-Id: If82e1fa29da464707604646e1d1cfa220b49a5ba
Merge "New template helper: escapeContent" into frontend-rewrite