Strip "Last updated at" tags on export

[dshoreman]

<time> tags throw an "Invalid entity" warning when we convert HTML to Markdown because DOMDocument is ancient, and the <small> tag isn't in Markdown either. The latter is solved by setting strip_tags, but it doesn't help the time issue.

We're already parsing these lines to save the edit time, so once #2 is solved let's strip them from the post as part of the export process.