Initiate Jira import on frontend

What does this MR do?

This MR allows the user to initiate a Jira import on the frontend.

If the user goes to the issues page and selects Import from Jira, they will land on a page which:

• Shows a Set up Jira integration screen if Jira integration is not configured for the project
• Shows an Import in progress screen if a Jira import is scheduled or has started
• Otherwise shows a Jira import form for the user to select which Jira project to import

If the user selects a Jira project to import and submits the form, this initiates a Jira import and the UI changes to show the user that an import is in progress.

Feature flags: :jira_issue_import and :jira_issue_import_vue

Issue: #208907 (closed)

Screenshots

Jira integration not configured	Screen_Recording_2020-04-13_at_10.40.14_am
Import in progress	Screen_Recording_2020-04-13_at_10.39.29_am
Initiate Jira import	Screen_Recording_2020-04-13_at_10.40.57_am

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