Replace import/export CSV buttons and modal with Vue component

What does this MR do?

This MR adds a new Vue component that renders import and export buttons as well as the modals on the following pages:

• Project issues
• Project merge requests

This component will also be added to an issue's empty page in a separate MR. The deprecated HAML templates will also be removed in the follow-up MR.

Screenshots (strongly suggested)

Issues

	Before	After
Buttons
Export Modal
Import Modal

Merge Requests

	Before	After
Buttons
Export Modal

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

Related to #254267 (closed)