[Part 1] Add Vue Issuable sidebar feature flag & app bundle

What does this MR do?

Background

This MR is a part of series of MRs related Right Sidebar refactor effort. Everything done in this MR is behind following group-level feature flags;

• :vue_issuable_sidebar - This flag controls feature visibility on Issue and MR pages.
• :vue_issuable_epic_sidebar - This flag controls feature visibility on Epic page.

You can enable feature flag locally for a specific group by running following commands;

# Open rails console

$ bin/rails console

# Enable feature flag

# replace path value with desired group path

> Feature.enable(:vue_issuable_sidebar, Group.find_by(path: 'gitlab-org'))

MR Summary

This MR adds feature flag for responsible controllers and adds bare-bones Vue Issuable Sidebar app under path app/assets/javascripts/issuable_sidebar which is initialized when feature flag is enabled (and it replaces older sidebar).

Screenshots

Conformity

• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance 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

#31910 (closed)