Move code to ~/issuable

What does this MR do and why?

Issue/MR code has recently been moved to ~/issuable to declutter the top-level ~/ directory and to co-locate code for better organisation.

This MR moves more issue/MR code to ~/issuable to organise and co-locate code. app/assets/javascripts/vue_shared/mixins/issuable.js was also deleted and its contents moved to app/assets/javascripts/notes/components/discussion_locked_widget.vue, the only file that imported it.

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MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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

Closes #239168 (closed)