Add jira association missing state to MR widget

What does this MR do?

Adds a jira association missing state to MR widget as described in #280766 (closed).

Setup and testing

1. Enable the development feature flag echo "Feature.enable(:jira_issue_association_on_merge_request)" | rails c
2. Create or view an existing MR in the project that does not have a Jira issue associated with it
3. Enable the setting ​Require an associated issue from Jira in Project > Settings > Merge checks
4. Reload the MR page to verify that the Merge button is now disabled

Screenshots (strongly suggested)

Setting disabled 🔴	Setting enabled ✅

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 it is behind a development 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

Related to #280766 (closed)