Verify that Webmention endpoint is well formed
To ensure that I don't accidentally break my Webmention discovery in the future, I want to use my newfound power with HTMLProofer's custom `Check`s to catch future regressions in the metadata. Note that we only check this on the home page as that's the important place that a client would be looking it up - other pages may be valid, but this is the key one. Closes #386.
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