Bootstrap applications without DOMContentLoaded

What does this MR do?

• Unwrap the code inside the DOMContentLoaded handlers in the following locations:
  • app/assets/javascripts/pages/projects/issues/index/index.js
  • app/assets/javascripts/pages/projects/issues/new/index.js
  • app/assets/javascripts/pages/projects/issues/service_desk/index.js
  • app/assets/javascripts/pages/projects/issues/show/index.js
  • ee/app/assets/javascripts/pages/projects/issues/show/index.js

Why?

Plese refer to &4538 (closed) and also an internal discussion

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done