Send grade scheme as a string

Summary

Turns the grade scheme into a string before sending it to receiving scripts.

Changes

Previously, Submit sent the grade scheme enum, which was serialized into a JSON string:

--I7NQh0kN3mm2LjP6qT-h6fUJwB7abayW
Content-Disposition: form-data; name="gradeScheme"
Content-Type: application/json

"PASS_FAIL"

In effect, the MIME type was wrong and the scheme was quoted. By sending the name of the enum, the unquoted, text/plain value is sent instead.

Does this MR meet the acceptance criteria?

• The main application was run to make sure the server still runs without errors.
• Tests were written to verify the behaviour of this code.
• Javadoc and other comments were added to make the code understandable.
• The code adheres to the style guidelines.