Add missing SPL outcome nodes

Use the outcome naming convention to derive potential snippets. Also
shows which things are applicable in this outcome
Add a TODO node, as a placeholder for missing step

This outcome can be used as a placeholder where the next question/outcome has not been
written, or is unknown. This allows next node rules to point at an outcome without
breaking the graph of the flow (which will be a linting error in the future)
Improve the SPL start page

Try to manage expectations a bit, for anyone working on the flow, or
testing it. Make clear it's not live yet
Support more question types in Smartdown visualisation
Show human forms of Combined/Comparison predicates on graph labels

Previously they showned objectect representations, which was a lot less useful.

It's real hacky doing it this way, but this was making quite a few graphs unreadable.

This fixes it for now, but this behaviour should proabbly be pushed down into
Smartdown and the predicates themselves
Merge pull request #1109 from alphagov/spl-flow-improvements

Spl flow improvements