docs: recent sphinx-build warns about 'rfc' role being overridden

[elliefm]

When building our docs with sphinx-build 1.4.5:

WARNING: while setting up extension ?: role 'rfc' is already registered, it will be overridden

Our rfc is defined in docsrc/conf.py:

        'rfc':('http://tools.ietf.org/html/rfc%s', 'RFC '),

The thing is, sphinx already has such a thing built in. And looks like it has had it built in pretty much forever: sphinx-doc/sphinx@d60ca8d

The difference with the newer version of sphinx-build appears to be that it now warns about us overriding built-in functionality, instead of just quietly doing it. We should be able to shut up this warning just by no longer defining our own rfc role. The built-in one does basically the same thing anyway (except that it uses a https link to the same destination -- good!).

We need to support at least back to version 1.3.6, because that's what the website rebuild uses, but I can't easily test that version now that I've upgraded.

[nicolan]

I'm running sphinx 1.3.6

I yanked the rfc entry in conf.py and everything built beautifully.

[nicolan]

I can't seem to assign it to me or mark it fixed.

commit 3a004c1

[elliefm]

@nicolan Awesome, thanks, #10 has been merged. I've sent you an invite to the cyrusimap group, once you accept it I think you'll be able to assign/close issues.

I'll leave this one hanging open for now so you can try it out

[nicolan]

Pull request merged. Closing issue.
And confirming permissions work!