Bug 1072159 - Emphasise treeherder URLs in Try emails & fix for try-comm-central; r=nthomas

* Emphasise treeherder over TBPL, since the treeherder transition is imminent.
* s/thunderbird-try/try-comm-central/ for treeherder, now that it uses the
  hg/buildbot name (bug 1035222).
* Remove unused import and fix flake8 whitespace warning.