LTI1.3 tool: Create lineitem if not found while sending grade

[murhum1]

Fix for the issue where after some moodle operations grades would no longer be sent from A+ to moodle. Now lineitem existence is checked before trying to send the grade, and is recreated if not found.

Fixes #1483

Testing

What type of test did you run?

• Accessibility test using the WAVE extension.
• Django unit tests.
• Selenium tests.
• Other test. (Add a description below)
• Manual testing.

Tested manually removing a lineitem created by A+ and trying to submit the associated exercise before and after changes, verifying that the error happened before the change and no longer after the change.

Also tested that no duplicate lineitems were created if one already existed, e.g. tested submitting without manually removing the lineitem beforehand, as well as doing multiple submissions to the exercise.

Did you test the changes in

• Chrome
• Firefox
• This pull request cannot be tested in the browser.

[ihalaij1]

I'm not familiar with this issue nor do I know how to test it. But looks like you tested it already so I'm going to approve.

Also, could you add Fixes #1483 to the commit message footer before merging?