Jira issues && GitLab Milestones improvements

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ZenDesk: https://gitlab.zendesk.com/agent/tickets/24337

User

When I create a new issue under a milestone using "+ New Issue" button, I get a html 404 page.

Expectations

1. When the new issue button under a milestone is clicked, I takes the user to create an issue on the linked jira installation
2. Under the Issues tab in a milestone, the user should be able to see the Jiras created under the milestone.

cc/ @JobV