Clean up (usage of) onResult, makeResult, and onNext

[drikusroor]

1. Remove onNext calls from the onResult callback function

We can probably just wait for onResult to finish and call onNext manually (in makeResult)

2. Remove result_id from the backend Trial constructor and the if (result_id) condition in Trial.tsx.

MUSCLE/frontend/src/components/Trial/Trial.tsx

Lines 76 to 83 in 00c3e63

	if (result_id) {
	onResult({
	result,
	result_id
	});
	} else {
	onNext();
	}

This bit is in here for "historical reasons" - the Trial has a result_id at top level, but this is not necessary anymore. (Before a refactor, the "breaking round" mechanism was handled through this top level result id.) I checked, and indeed, no Trial in the current rules files contains a result_id. We should remove result_id from the backend Trial constructor and this condition here. (Can be left for later though.)

Originally posted by @BeritJanssen in #1254 (comment)

UPDATE

Some work in progress can be found in #1258