Detect when a site no longer has MW at the URL

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Sites like this are now running WordPress and are ending up in lots of logs like "Unhandled exception 'NoneType' object has no attribute 'group' during collect statistics (API)".

We should verify that MediaWiki is still there, first.

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This issue was highlighted when I received the following report from BitNinja.io

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Note that Linode support classified this as a TOS violation.

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Looks like @thingles created audit-bee (logs) for this purpose.