MB-8740: updated Forum link to Community Portal

Change-Id: I43711a84bf5043b101bfe7ce8c4b0ba6482bd8fa
Reviewed-on: http://review.couchbase.org/28351
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>