Convert Issue import button to dropdown

What does this MR do?

This MR converts the Issue import button to a dropdown to include a Import from Jira link.

The changes are done in HAML because none of the buttons, form inputs and dropdowns in the issues page are in Vue and I didn't want to hook in some Vue just for a single button dropdown.

This change uses feature flag :jira_issue_import which is defaulted to off

Issue: #208907 (closed)

Screenshots

before	after

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