Update webpack to 3.6.0

Update react-contextmenu to the latest version 

Extension library

Alphabetize all the things

Feature/extensions

This should fix the integration test failures on the extensions branch.
Thanks @paulkaplan for figuring this out!

Add minimum trimmable length for audio trimmer

Fix travis problems

Fix eslint error from prop types

Update react-draggable to the latest version 

Update jest to the latest version 

Update react-intl to the latest version 

Update react-tabs to the latest version 

Update react-modal to the latest version 

Do not render invisible components

chore(package): update css-loader to version 0.28.7

Closes #656

To show the "Add Extension" button, you must now add `?extensions` to
the URL used to load the GUI.

Also, delete `static-blocks.xml` which shouldn't have been added in the
first place.