Consolidate logic tree service endpoints

The LogicTrees and current Features endpoints both return tree structures, one for MFDs and one for features (modified, stitched, and recombined from their original definitions, see #539 (closed)). Combining these will free up the features endpoint path for #539 (closed).

changed milestone to %NSHM Model and Tool Improvements

added ERF Services labels

assigned to @pmpowers

mentioned in issue nshmp-apps#162 (closed)

mentioned in merge request !671 (merged)

mentioned in commit b59387fa

closed with merge request !671 (merged)