Add "Link" functionality to Jira Connect app new UI

What does this MR do?

This is the last MR after !51061 (merged) to add a new UI for our Jira Connect app. This feature is behind the feature flag new_jira_connect_ui added in !50692 (merged).

This MR adds a "Link" button next to each group (or namespace) which allows the user to subscribe that GitLab namespace to the Jira Connect app.

Previously, users had to manually key in the group path in a text input and click a button. We now show a button that opens up a modal with a list of groups the user has access to (and are not already linked). Users can then click on the "Link" button to add a group.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry - feature flag
• [-] Documentation ([if required] (https://docs.gitlab.com/ee/development/documentation/workflow.html#when-documentation-is-required)) - feature flag
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

Related to #13020 (closed)