Fixing up group homepage problem (Bug #1396565)

A mistake was made where a user was allowed to add more than one
"group pages" block to the group homepage. This wasn't a problem until
we added pagination to the "group pages" block.

This patch attempts to clean up that error and merge any existing
multiple block instances into one "group pages" block instance.

Change-Id: I38acada835a4c245c97f0008543519a9dd2072a3
Signed-off-by: Robert Lyon <robertl@catalyst.net.nz>