Move JS code into ~/issuable and ~/issues

What does this MR do and why?

This MR moves JS code into ~/issuable and ~/issues

• Move into ~/issues because they are only used on issue pages
  • issue.js
  • manual_ordering.js
• Move into ~/issuable because they are shared between issue and merge request pages
  • issuable_template_selector.js
  • issuable_template_selectors.js

Screenshots or screen recordings

How to set up and validate locally

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.