SSP-1701 Don't display step/detail nodes until entire tree loaded

This is just for the step/detail selector in the advisor-facing
Journal tool. Doesn't affect the admin Journal management UIs.

Also added a loading mask during the load/tree building process.

Doesn't deal well at all with any errors that might occur during
loading. But that problem was already present. You'd just get
a partial tree. Now you won't get anything at all (loading mask
won't dismiss).