Add developer mode that shows UUIDs of elements

[lukasrad02]

When using logging or inspecting objects during debugging, you often only see UUIDs of elements.

Therefore, it would be handy if the frontend can be configured to show element UUIDs in their popups or show the UUIDs by default when running in development mode (determine by NODE_ENV). If it is not determined from the environment, this setting should not have to be set every time the page loads but be persisted across page loads since the page reloads frequently during development.

[anonym-HPI]

This whole thing is in the end just a frontend problem, so:
What about instead of an ENV we could use something when an exercise ID is entered and the exercise ID is something like 12345678 for a trainer, we could use something like 12345678#debug to enable debug mode or just write #debug behind the url. This should be then be kept even on reloads.
And # things shouldn't be sent to the server and be just locally in the browser.

This could later be used for other debug stuff or different verbose modes of debug.