MB-8797 Hide "Generate Diagnostic Report" for read-only user.

Change-Id: I130352f6721f0fbbf869b5dac5d25c74a2202f86
Reviewed-on: http://review.couchbase.org/28240
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>