Jest takes an argument to specify which tests should be run. The
argument is treated as a regular expression and matched against the
filenames of potential tests. Unfortunately Jest does not normalize path
separators to forward slashes when running this comparison, so (for
example) `tests/unit` doesn't match anything on Windows.

This change replaces the forward slash in Jest arguments with `[\\\\/]`.

Ideally, that should just be `[\\/]` -- escaping the backslash so that
it survives NPM's JSON parsing -- but unfortunately the Windows version
of Jest parses its command line arguments differently from other
platforms. Specifically, the Windows version of Jest processes
backslashes one extra time. Fortunately having a redundant backslash is
OK on Mac and Linux, so `[\\\\/]` ends up working on all platforms.

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.