markdown: Cache Twitter 403 responses that are semi-permanent.
markdown: Cache Twitter 403 responses that are semi-permanent.

03ca3afbc2 added more codes that are equivalent to 404's; this adds to
the list of cache-as-None codes a couple which are equivalent to
403's.  It does not comprise _all_ possible 403-like codes -- many of
them are "the client is not OK," which is relevant to log as an error
still.

showell/zulip,andersk/zulip,rht/zulip,kou/zulip,eeshangarg/zulip,punchagan/zulip,andersk/zulip,zulip/zulip,hackerkid/zulip,zulip/zulip,kou/zulip,rht/zulip,andersk/zulip,showell/zulip,hackerkid/zulip,eeshangarg/zulip,andersk/zulip,showell/zulip,kou/zulip,punchagan/zulip,rht/zulip,punchagan/zulip,hackerkid/zulip,andersk/zulip,rht/zulip,punchagan/zulip,zulip/zulip,zulip/zulip,hackerkid/zulip,kou/zulip,hackerkid/zulip,showell/zulip,showell/zulip,kou/zulip,eeshangarg/zulip,zulip/zulip,rht/zulip,eeshangarg/zulip,kou/zulip,andersk/zulip,andersk/zulip,rht/zulip,rht/zulip,punchagan/zulip,hackerkid/zulip,showell/zulip,kou/zulip,eeshangarg/zulip,showell/zulip,punchagan/zulip,zulip/zulip,eeshangarg/zulip,eeshangarg/zulip,zulip/zulip,hackerkid/zulip,punchagan/zulip