Organise ~/issuable code

What does this MR do and why?

This MR moves init functions in ~/issuable into index.js and deletes issuable_index.js as it is not a useful extra class.

• Init functions are conventionally in an index.js file so this MR moves init functions into index.js for consistency
• Having a file for every init function can cause clutter in a directory which adds a bit of friction when navigating/working with the code. Having one index.js to hold all init functions for a module makes the code more organised, in the same way that having one utils.js file to hold all util functions for a module makes the code more organised.

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