Finding areas that are untested and need love (#4131)

* Makes InstallationRouter like others

* Adds testing for Range file requests

- Fixes issue with small requests (0-2)

* Revert "Makes InstallationRouter like others"

This reverts commit e2d2a16ebf2757db6138c7b5b33c97c56c69ead6.

* Better handling of errors in FilesRouter

* Fix incorrectness in range requests

* Better/simpler logic

* Only on mongo at it requires Gridstore

* Open file streaming to all adapters supporting it

* Improves coverage of parsers

* Ensures depreciation warning is effective

* Removes unused function

* de-duplicate logic

* Removes necessity of overriding req.params.className on subclasses routers

* Use babel-preset-env to ensure min-version compatible code

* removes dead code

* Leverage indexes in order to infer which field is duplicated upon signup

- A note mentioned that it would be possible to leverage using the indexes on username/email to infer which is duplicated

* Small nit

* Better template to match column name

* Restores original implementation for safety

* nits

src/Routers/RolesRouter.js

@@ -2,29 +2,8 @@
 import ClassesRouter from './ClassesRouter';
 
 export class RolesRouter extends ClassesRouter {
-  handleFind(req) {
-    req.params.className = '_Role';
-    return super.handleFind(req);
-  }
-
-  handleGet(req) {
-    req.params.className = '_Role';
-    return super.handleGet(req);
-  }
-
-  handleCreate(req) {
-    req.params.className = '_Role';
-    return super.handleCreate(req);
-  }
-
-  handleUpdate(req) {
-    req.params.className = '_Role';
-    return super.handleUpdate(req);
-  }
-
-  handleDelete(req) {
-    req.params.className = '_Role';
-    return super.handleDelete(req);
+  className() {
+    return '_Role';
   }
 
   mountRoutes() {