feat(plugins): Add initial version of ViewportPlugin class (#242)

* Add initial version of ViewportPlugin class
* Move entrypoints from OHIFPlugin to OHIF.plugins
* Add URL route to DICOMWeb server for Standalone Viewer

Packages/ohif-viewerbase/client/components/viewer/imageControls/imageControls.js

@@ -65,8 +65,8 @@ Template.imageControls.events({
             // Using the slider in an inactive viewport
             // should cause that viewport to become active
             const $slider = $(event.currentTarget);
-            const newActiveElement = $slider.parents('.viewportContainer').find('.imageViewerViewport');
-            setActiveViewport(newActiveElement);
+            const viewportContainer = $slider.parents('.viewportContainer');
+            setActiveViewport(viewportContainer);
 
             // Subtract 1 here since the slider goes from 1 to N images
             // But the stack indexing starts at 0