[site] Update slack invite link - take 2

This commit updates the broken slack invite link in all the
locations where it is being used. This link works for 400
users after which it needs to be regenerated and updated.

Change-Id: Id3090bcb452c72873fedd32b2b3074ebd99b3f87
Reviewed-on: http://gerrit.cloudera.org:8080/20598
Reviewed-by: Alexey Serbin <alexey@apache.org>
Tested-by: Abhishek Chennaka <achennaka@cloudera.com>
3 files changed