Add comments to Jira issues

What does this MR do?

Renders comments on Jira issues. Notable changes include:

• extended issuable_show_root.vue component to include a discussion slot
• modified shape of comments data in API to be more consistent with our existing notes API

⚠ This branch is currently targeting add-jira-note-component (!56068 (merged))

⛳ This feature is currently behind the :jira_issues_show_integration feature flag, disabled by default

Related issue: #322638 (closed)

Iteration	MR
1. Add note.vue component	!56068 (merged)
2. Render comments on Jira issues within GitLab	You are here 🚀

UX approval was given in !55814 (comment 523526607)

Screenshots (strongly suggested)

Large	Small	With long user name

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because this is behind a feature flag.
• 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

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team