Fix problem being graded after focusing remark text area

Closes #279

See merge request zesje/zesje!120

Add keyboard shortcuts for grading

Closes #55

See merge request zesje/zesje!113

Altering the keybindings requires non-local changes, because the
tooltips must also be updated, so we add a comment to ensure we
won't forget it.

use a ref instead of id for focus management

Closes #282

See merge request zesje/zesje!118

Closes #282. Refs should be used with caution, but focus management is the main
suggested application, see https://reactjs.org/docs/refs-and-the-dom.html

Add a message when student has multiple submissions

Closes #281

See merge request zesje/zesje!117

add tests for scan pipeline

See merge request zesje/zesje!107

Autofocus student matching

See merge request zesje/zesje!114

Fixing lint errors

See merge request zesje/zesje!115