improve: lazy resolver (#4253)

* default to empty `ImmutableMap` when grabbing op metadata
* pass `errors` into JsonSchema components
* Account for Immutable data structure in JavaScriptonSchema...
...and create empty Lists instead of Maps by default.
* Pass ImmutableList through to JsonSchema child components
* Add lazy resolving spec state extensions
* TEMPORARY: disable conventional resolved spec
* WIP
* Use resolveSubtree in Operation display
* Freebie: short-circuit Markdown component if it is given plaintext
* NEW DEFAULT BEHAVIOR: `defaultModelsExpandDepth: 1` does not expand individual models
* Render faked Model expander to trigger resolution
* Baseline support for Editor lifecycles
* Display operation summaries before the operation is resolved
* Test migrations
* WIP
* Swagger2 TIO Body params
* a bit of cleanup
* Debounce string param inputs
* Reach into unresolved operation for deprecated flag, if available
* Fire subtree request outside of render
* Remove debugging flags
* Fix logical errors in spec statePlugins
* TODOs become TODONEs!
* Migrate deeplinking feature to non-resolved spec action
* ESLint fixes
* Reduce action dispatch volume; run resolver on next tick
* Batch resolver requests; use batch progressive results in later iterations
* Add loading states to Model and Operation views
* Preserve object order; remove dupl. import; add warning for action
* LINTER!
* Use require to load SVG
       Works around Webpack weirdness

src/style/_layout.scss

@@ -586,6 +586,16 @@
     }
 }
 
+.opblock-body
+{
+  .opblock-loading-animation
+  {
+    display: block;
+    margin: 3em;
+    margin-left: auto;
+    margin-right: auto;
+  }
+}
 
 .opblock-body pre
 {