[PROPOSAL] Remove nabble links from website

Hi guys,

I created pull request 
https://github.com/apache/activemq-website/pull/56 on website to remove 
the nabble link from the contact page.

Since nabble downgraded their servers capacity, a bunch of forums linked 
to mailing list are not available anymore. It's the case for several 
Apache projects, including Apache ActiveMQ.

Users should use mailing list and eventually slack (but remember, if it 
doesn't happen on the mailing list, it never happens, so discussions on 
slack should moved to mailing list at some point).

If there's no objection, I will merge PR #56.

Thanks,
Regards
JB