#567 fix wrong name as last grader being displayed in grading form

Instead of the user who graded the submission as last the current user got displayed in the grading form.
Additionally improved some parts using $USER when displaying feedback for a submission.
peer review comments
#526 replace double quoted strings with single quoted ones

To maintain/improve compatibility with moodle coding standards
we replaced every possible double quoted string through
it's single quoted equivalent.
#524 remove unused variables and methods/functions
#529 move checkmark grading form to a separate file
#530 use symbol's codes instead of symbols themselves for (un)checked examples
#525 avoid some smaller repeating code segments
#524 adapt code to moodle standards (and --> &&, globals on top)
#250 redesign DB-structure and connected methods/functions