feat(multi): add editor decoration

The warning region does not grow at the edges This makes it harder for
the user to get into an odd state.  It might be better to have let the
user expand the second region at the trailing edge.  I'm not sure, and
that's harder to implement.

client/src/templates/Challenges/redux/index.js

@@ -106,6 +106,7 @@ export const sagas = [
   ...createCurrentChallengeSaga(types)
 ];
 
+// TODO: can createPoly handle editable region, rather than separating it?
 export const createFiles = createAction(types.createFiles, challengeFiles =>
   Object.keys(challengeFiles)
     .filter(key => challengeFiles[key])
@@ -115,7 +116,8 @@ export const createFiles = createAction(types.createFiles, challengeFiles =>
         ...challengeFiles,
         [file.key]: {
           ...createPoly(file),
-          seed: file.contents.slice(0)
+          seed: file.contents.slice(0),
+          editableRegion: file.editableRegion
         }
       }),
       {}
@@ -311,7 +313,8 @@ export const reducer = handleActions(
               ...files,
               [file.key]: {
                 ...file,
-                contents: file.seed.slice()
+                contents: file.seed.slice(),
+                editableRegion: file.editableRegion
               }
             }),
             {}