* Ensure that when you open the explorer the focus moves to the first link of the menu, and add a label for the explorer navigation
* Add dialog role to pages explorer popup, and ensure that there is an option to close the dialog window when tabbing by making the close button visually hidden rather than display none
* Adds clearer text to the close button for modals
* Use existing wagtailConfig.STRINGS to store the new translatable string for JS, rather than introducing a new library.
* Update list of string constants for translation in the tests file
* Remove aria-hidden on modal dialog close button
Instead of passing an 'onload' JS function as part of the AJAX response for each step of the workflow,
we specify all onload handlers up-front when initialising ModalWorkflow, and return a 'step' field
in the response to indicate which one to trigger.
* Expose Draftail package as global variable for reuse
* Expose Wagtail React components for reuse
* Expose Draftail-related React components for reuse
Updated snapshots due to the upgrade and failing ExplorerPanel test. This was due to the shallow call being made earlier than defining the document.body
Update to React 16
Update other dependencies
Stylelint updates
Remove unused imports
Update babel and gulp packages
Update package-lock
