have only one 'create issue' button

Formerly, if an app had a github repo, but another issue tracker was
configured, there would be two 'create issue' buttons (one for github
and one for the other issue tracker.) If there is an issue tracker
configured, we should use only that one.
Add spec for #655

Hint: This diff is much easier to read if you ignore whitespace
Merge pull request #655 from brandedcrate/features/extract_issue_tracker-one_issue_button

have only one 'create issue' button