Allow changing cluster ram size from UI

The admin UI allows administrators to change the per-node RAM quota for a bucket, but does not allow changing the RAM size for the whole cluster. This can be done via the command line (/opt/membase/bin/membase or /opt/couchbase/bin/couchbase-cli) with the cluster-init --cluster-init-ramsize=NNNN option. It would be nice to have this available in the UI (perhaps with an appropriate "Are you sure?" modal dialogue box to avoid casual mistakes.