Auto-start JS modules matching a data pattern

* Find and start modules matching the pattern [data-module], eg.
data-module=“selectable-table”
* Look for modules in a register, GOVUK.Modules
* Require module instantiation before starting
* Convert module names to camel case to match JS syntax
* Create a GOVUK ‘app’ which runs on page load, sets up the necessary
structures and does the finding and starting of modules
* Update selectable table to match this pattern
* Remove the old instantiation and checks that are no longer needed
Split mappings into two modules

* Create toggle and toggle form fields modules, using new module pattern
* Remove inline script calls to Mappings.edit()
* Remove a redundant toggle module on bulk adder, nothing was being
toggled
Move modules into subdirectory

* Keep them distinct from other JS files
Rename pattern to moduleSelector

* It’s a selector, not a regex
Merge pull request #192 from alphagov/js-auto-start

Auto-start JavaScript modules that appear in markup