REST: allow fetching of subject prefixes

REST: allow fetching of subject prefixes

Some mailing lists accept patches for multiple projects, and use
a subject prefix to differentiate the projects.

Therefore, for snowpatch, it's useful to be able to fetch the
subject prefixes.

Export subject prefixes in the REST API.

Signed-off-by: Daniel Axtens <dja@axtens.net>
Tested-by: Russell Currey <ruscur@russell.cc>
Reviewed-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com>
Reviewed-by: Stephen Finucane <stephen@that.guru>