Cleanup Banzai ExternalIssueReferenceFilter

What does this MR do?

It cleans the ExternalIssueReferenceFilter class by adding internationalization, extracting some parts into methods and make internal methods private.

How to test on staging

1. Set up a project with Jira integration and reference a Jira issue from a GitLab issue.
2. Make sure the link works
3. Make sure when you hover over the link, the title box says "Issue in Jira"

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] 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