New image measurement handler, Heap Analytics user tracking added and default permissions warning removed

Today's update (e9988b9) brings with it the following changes:

New features

• New request handler for images was added that make it possible to quickly measure the dimensions of a rectangle in an image which can be used directly in the findinpicture app. This handler is available in the edit top menu. A copy-to-clipboard button is also available which is focused after a selection is done. Some SVG images are not compatible with this rendering method and can't be properly measured. This issue might be fixed in a later update.
• Added user client-side event tracking using HeapAnalytics.com. This can be selectively enabled for a site by setting the heap_analytics_id configuration variable.
• When you're creating exercises (app-input objects) it is now possible to quickly look up the app you want to use with the app selector located next to the custom_init input box in the object editor.

Enhancements

• The list_config request handler has been completely rewritten to use the new list_config template function.

Bugfixes

• When new objects were created a warning about broken default permissions was shown. This should no longer happen if no default permissions are set.

Feature removals / deprecations

• The object attributes structure, version and aggregation_level was removed, along with their backing database columns. They have never been used for anything useful. They were removed to save space and decrease user interface complexity.