Put each bit of spec in its own directory

I want to change the URLs for the spec sections on the website from
<version>/<section>.html to <section>/<version>.html, to better reflect how we
do the versioning.

This puts each bit of spec in its own directory, updates the index to point to
the right place, and fixes continuserv to deal with directories as well as
files.

This will probably require fixes to the speculator too, but I'll have to come
back to that.

specification/targets.yaml

@@ -12,19 +12,24 @@ targets:
       - { 1: modules.rst }
       - { 2: feature_profiles.rst }
       - { 2: "group:modules" }  # reference a group of files
+    version_label: "%CLIENT_RELEASE_LABEL%"
   application_service:
     files:
       - application_service_api.rst
+    version_label: unstable
   server_server:
     files:
       - server_server_api.rst
       - { 1: event_signing.rst }
+    version_label: "%SERVER_RELEASE_LABEL%"
   identity_service:
     files:
       - identity_service_api.rst
+    version_label: unstable
   push_gateway:
     files:
       - push_gateway.rst
+    version_label: unstable
   appendices:
     files:
       - appendices.rst