urls: Encode slashes in message IDs

urls: Encode slashes in message IDs

We were attempting to work around the fact that message IDs could
contain slashes which in some cases broke our ability to generate
meaningful URLs. Rather than doing this, insist that users encode these
slashes so that we can distinguish between semantically meaningful
slashes and those that form the URL. This is a slightly breaking change,
but the current behavior is already broken (see the linked bug) so this
seems reasonable.

Signed-off-by: Stephen Finucane <stephen@that.guru>
Closes: #433
Cc: dja@axtens.net
Cc: siddhesh@gotplt.org