Introduce separate grading view

[troeger]

This view should be independent from the editing of submissions. The view should be offered in the teaching backend as separate activity, e.g. with a new button in the last column.

The view should only show grading-related information. The grading status should be mapped to according "Save as unfinished" and "Save as finished".

It should be easily possible to go to the next gradable submission. This could be driven by the submission list filter that was active before.

Having such a view is the precondition for showing submission-specific duplicate report and MOSS details (see #67).