Genericize design docs directory.  #21.

Other types of design docs will soon be stored here.
HistoryDb -> Couchdb.

The CouchDB instance used by ArchiveBot will soon contain named ignore
sets.  While it is true that this turns the CouchDB instance into a
dumping ground for data that we want to be persistent, it's also the
truest representation of what we use CouchDB for, so meh.