Remove code from global namespace

What does this MR do?

1. Removes the following from global namespace & enables eslint:

• IssuableContext
• IssuableIndex
• IssuableForm
• ImporterStatus

1. Removes old non used code from IssuableForm

2. Fixes broken dependencies in tests

3. Adds better tests fro IssuableForm

4. Remove iife from tests

Are there points in the code the reviewer needs to double check?

Yes. A lot of code was removed from IssuableForm. We need to double check if it is 100% safe to do it.

Does this MR meet the acceptance criteria?

• Tests added for this feature/bug
• Review
  • Has been reviewed by Frontend
• Conform by the merge request performance guides
• Conform by the style guides

What are the relevant issue numbers?

#38869 (moved)