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/core/components/model.jsx

@@ -49,6 +49,17 @@ export default class Model extends ImmutablePureComponent {
       schema = this.getRefSchema( name )
     }
 
+    if(!schema) {
+      return <span className="model model-title">
+              <span className="model-title__text">{ name }</span>
+              <img src={require("core/../img/rolling-load.svg")} height={"20px"} width={"20px"} style={{
+                  marginLeft: "1em",
+                  position: "relative",
+                  bottom: "0px"
+                }} />
+            </span>
+    }
+
     const deprecated = specSelectors.isOAS3() && schema.get("deprecated")
     isRef = isRef !== undefined ? isRef : !!$$ref
     type = schema && schema.get("type") || type