indexes: add an index needed for message checking
admin-modules page: list modules and their versions
edit_clarification: fix a regex parse error when creating a clarification

caused by trying to "release the submission"
when we don't even have a submission id yet
templates: use call_url(...) to have a &next= argument be added to the url

never automatically add the next= argument as it cannot always
be predicted when it will be desired
implement deletion of clarifications and submissions
define_user: more thorough deletion of user object
define_system_test: partly implement deletion of system tests
define_problem: initial implementation of delete_problem
redirects: use absolute urls whenever redirecting