Add feature flag and boilerplate code for new Jira Connect app UI

What does this MR do?

As suggested by @leipert, I have extracted the addition of the feature flag and the basic Vue / GitLab UI boilerplate code from the original MR !49202 (closed) to get these changes merged faster.

Since most changes are behind a feature flag, there shouldn't be any big changes to the UI (apart from some UI refinements which are intentional).

Changes in this MR include:

• Add feature flag new_jira_connect_ui and pass it to the frontend. When ff is enabled, show some new UI elements and hide / remove unused ones.
• Add boilerplate JS for Vue to allow using translations in templates and using feature flags. Since this is a completely new entrypoint, we need to initialize these separately.
• Add CSS used by GitLab UI and allow using GitLab UI utilities. Also make small style refinements to follow designs.

Screenshots (strongly suggested)

Before	After (without ff)	After (with ff)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry - Not needed since 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

Related to #13020 (closed)