Get collection usage size with ARM metrics call (#327)

- Removed `readCollectionQuotaInfo` call. The only data we need is the usage size which we can get via the ARM metrics call instead.
- Added `getCollectionUsageSize` which fetches the `DataUsage` and `IndexUsage` metrics, converts them to KB, and returns the sum as the total usage size

src/Explorer/Tree/Database.ts

@@ -193,7 +193,7 @@ export default class Database implements ViewModels.Database {
     });
 
     deltaCollections.toAdd.forEach((collection: DataModels.Collection) => {
-      const collectionVM: Collection = new Collection(this.container, this.id(), collection, null, null);
+      const collectionVM: Collection = new Collection(this.container, this.id(), collection);
       collectionVMs.push(collectionVM);
     });