core.models.candidate: Simplified, removing ALL cache fields.

Also removed the AbstractIsAdmin, the post_save handlers used to update
the cache fields and the etag field.
core...test_candidate: Removed. None of the tests are needed after

the simplifications in c2113b782e7fff83418b39f460ee9a4ef09f613b.
core.tests: Removed the imports in __init__. Not needed by django 1.6.
core.testhelper: Updated for the removal of Candidate.update_identifier.
devilry.apps.student: Removed.

It is no longer in use.
devilry_settings: Removed devilry.apps.student.
core...test_testhelper: Removed the test that used the removed

Candidate.identifier field.
devilry_settings.default_urls: Removed the old student, administrator and

examiner urls.
core...test_assignment_group: Removed the old test for update_identifier.
devilry_settings.default_settings: Removed the old examiner

and administrator apps.
devilry.apps.examiner: Removed. No longer in use, and it is broken.
devilry.apps.administrator: Removed. No longer in use, and it is broken.
devilry.apps: Removed the old gradeeditors module and all its plugins.
Removed all the old gradeeditors from settings.
devilry_settings.default_urls: Removed gradeeditors from URLs.
devilry_subjectadmin.rest.passedinpreviousperiod: Commented out the

imports of devilry.apps.gradeeditors. This breaks the API, but it needs
an update to work with the new gradingsystem API.
core.models.assignment_group: Removed get_candidates().

It is still used in some old modules, but I think we will end up
removing them, so not fixing them for now.