plagiarism_turnitin: Add function to check that user has accepted EULA

Signed-off-by: Tony Butler <tonyjbutler@gmail.com>
mod_turnitintooltwo: Check connection to TII before verifying EULA

Signed-off-by: Tony Butler <tonyjbutler@gmail.com>
mod_turnitintooltwo: Don't simply reload page on EULA acceptance

Either hide the warning message or re-submit the form,
depending on whether submission has already been attempted.

Signed-off-by: Tony Butler <tonyjbutler@gmail.com>