Cache flows in production and by config switch

Adopts a similar pattern used in the existing Smartanswers::FlowRegistry
class whereby production defaults to caching flows and options passed to
registry control the caching of flows.
Rename constant for use in both birth and death flows
Fact check changes for register a birth flow, content and tests
Fact check changes for register a death flow, content and tests
Merge pull request #1116 from alphagov/cache_smartdown_flows

Cache flows in production and by config switch
Merge pull request #1117 from alphagov/register-birth-death-fc-updates

Update register a birth and death from fact check changes
Always return an array of flows
Merge pull request #1118 from alphagov/always_return_an_array_of_flows

Always return an array of flows