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/Controls/Settings/SettingsSubComponents/ScaleComponent.tsx

@@ -186,7 +186,7 @@ export class ScaleComponent extends React.Component<ScaleComponentProps> {
       onScaleSaveableChange={this.props.onScaleSaveableChange}
       onScaleDiscardableChange={this.props.onScaleDiscardableChange}
       getThroughputWarningMessage={this.getThroughputWarningMessage}
-      usageSizeInKB={this.props.collection.quotaInfo().usageSizeInKB}
+      usageSizeInKB={this.props.collection.usageSizeInKB()}
     />
   );