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.
make the icons appear for available issue trackers

This fix is a little ugly because it requires instantiating a bunch of
useless tracker objects since the label can only be attained through an
instance method of the tracker object. It could be made much nicer if
the label were a class method instead.
Merge branch 'features/extract_issue_tracker-css_images'
Merge branch 'features/extract_issue_tracker-one_issue_button'