Implement support for takedown notices (infra, admin tools, workflow)

Takedown notices are coming, and we need full support to dereference certain contents, like it happens on https://github.com/github/dmca.

This involves several subtasks:

low level support for blacklisting specified contents (not only URLs, also SWHIDs), with support for regexps
admin interface to add/remove entries from the blacklist
a journal of these operations (what was added/removed, when and why, from the blacklist)
a public webpage that maintains the list of accepted takedown notices

Migrated from T3087 (view on Phabricator)

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