Change registered/enabled block to load as List rather than Set

Enabled blocks were being loaded as Set type. Since Sets are not ordered the, order of blocks is not guaranteed. Using this information in a UI will mean inconsistent appearance across restarts. There is a requirement in SLA-10754 to make the add-context-menu be ordered. The most straight forward way is to rely on the appearance of the blocks as listed in application config file. And making it load as a List for consistent ordering.

Closes SLA-10754