Debounce grade clicks

[niknetniko]

This pull request adds a debounce to saving the grade values, to fix #2821.

We'll probably need to play with the delay a bit, since it is a compromise between not waiting too long after typing a value, and waiting long enough that the debounce is useful when using the increment buttons.

Todo:

• Fix the system tests; they don't seem to understand the debounce.
• See if @chvp's suggestion to show the "saving" animation without delay (if the delay is so long that users think nothing happens) is needed

In the end I did opt to include the debounce from lodash:

• it's just the debounce, so it doesn't that large
• seemed unnecessary to write it ourselves
• it's already included by other dependencies, so there is no actually added code

This did require the allowSyntheticDefaultImports to be enabled, but I couldn't find any downsides. (and typescript in my ide kept complaining)

[bmesuere]

I think we already use a similar function: https://github.com/dodona-edu/dodona/blob/develop/app/assets/javascripts/util.js#L9

[niknetniko]

I've updated the code to use the existing utility function instead.