Add alert on project issues page to show Jira import has finished

What does this MR do?

This MR adds an alert on the project issues page to show the Jira import has finished. If the user has imported a Jira project into their GitLab project, then when it has finished an alert will appear on the project issues page to tell the user the import has finished.

This change is behind feature flag :jira_issue_import which is defaulted to on.

Issue: #215542 (closed)

Screenshots

Screen_Recording_2020-05-15_at_2.05.39_pm

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