Use get_or_create rather than get, by hacking a dependency

This fixed a crash experienced by Alex Rudnick. Props to him.
Changes from deployment
Add missing dependency to mysite.customs.tests
Access TrackerModel directly, rather than using a from import
Rename get_by_name to get_instance_by_name

This way, we can add a get_class_by_name method, which will
be used to find the corresponding TrackerModel object.

This is part of the plan to stop using core_bugimporters.all_trackers.
Refer to GitHubTrackerModel directly
Adjust whitespace so Emacs is happier
Copy TrackerModel metadata into each TrackerModel subclass

Also expose the metadata via get_* methods, for the ones that need to
import things.
Add get_by_name method

This way, we can stop looping over core_bugimporters.all_trackers.
Fix TrackerModel.get_edit_url to use self.short_name
Add get_tracker_model_or_404 helper

This permits the various views to share the logic.
Adjust views to stop using core_bugimporters.all_trackers

This is the minimal diff that seems to do the job. Note that some "if True" type noise
remains to avoid making the patch seem longer by fiddling with whitespace.
Remove core_bugimporters.all_trackers

Since this data has safely moved into the TrackerModel subclasses, and the views
have been updated, it is no longer needed here.
Trivially clean up some of the bug tracker edit views
Fix accidental bad copy paste
Use get_tracker_model_or_404 rather than NameError

Oops.
Remove numerous if True bits

These can safely be removed since the function stops executing in the case of a
tracker model being absent, due to the 404 getting raised.
Merge branch 'master' of git://github.com/openhatch/oh-mainline