stop fetching exam every time we graded: this is too slow

Closes #294

See merge request !130

Closes #294. Unfortunately now the feedback options will not be updated as often.

turn off caching for exported files

Closes #269

See merge request !129

Closes #269

Fix typo and update outdated exam finalization text

See merge request zesje/zesje!128

Only mark problems as graded when feedback options are selected

See merge request zesje/zesje!127

Human readable graded by string

Closes #278

See merge request zesje/zesje!125

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 !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