Implement 404 testing strategy

[techtonik]

How do we do 404 testing? It looks like we don't have automation to back this up and run at least monthly.

(if we do have this strategy, this should be put into some entrypoint Maintenance Quests document)

[techtonik]

Features creeping:

• stats on redirects from old moved pages (needed for fixing old links on the internets and to estimate if the redirect endpoint should be preserved)

[chadwhitacre]

We check for 404s in TestPages.browse, but I don't believe we do any external link checking.

[chadwhitacre]

You want 404 checking on Inside Gratipay, right?