Move to msgid based URLs

Move to msgid based URLs

Migrate our URL schema as follows:

Patches:       /project/<linkname>/patch/<msgid>/
Cover Letters: /project/<linkname>/cover/<msgid>/

The usual sub-resources (mbox, raw) hang off those URLs.
The old style URLs (/patch/NNN/*, /cover/NNN/*) redirect appropriately.

I haven't attempted to do anything meaningful with series, and I
have dropped any attempt to provide a generic message-id lookup
or search functionality. One step at a time.

Our database still stores message ids as with angle brackets; we
just work around that rather than trying to migrate. That too can
come later if we think the pain is justified.

Partially-closes: #106
Reported-by: Konstantin Ryabitsev <konstantin@linuxfoundation.org>
Reported-by-but-I-don't-want-to-spam: Linus Torvalds <torvalds@linux-foundation.org>
Reported-by: Stephen Finucane <stephen@that.guru>
Signed-off-by: Daniel Axtens <dja@axtens.net>