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