Prevent two issue trackers services being activated at same time
Summary
When you activate two issue tracker services, for example, Redmine and Jira, Gitlab only
considers the last one activated. We should validate activation of two issue trackers.
Steps to reproduce
Go to project services and activate Jira service Still at project services activate Redmine
Expected behavior
Application should prevent Redmine from being activated.
Actual behavior
Redmine gets activated and ignores Jira configuration which is still active,
when the user click on issues on project page it gets redirected to last configured service.
This is a minor issue but it is better to fix it to keep a consistent application.