Fix mergeUsers mutation with regard to answers and other constraints

[caillou]

As of now, the userId of an answer can not be modified due to the following PG trigger:

backends/packages/voting/migrations/sqls/20190907043631-questionnaire-improvements-up.sql

Lines 20 to 26 in d390713

	IF (
	TG_OP = 'UPDATE' AND
	NEW."userId" IS NOT NULL AND
	NEW."userId" != OLD."userId"
	) THEN
	RAISE EXCEPTION 'PSQL EXCEPTION: answers must not switch users';
	END IF;

A possible workarround could be this: 0ad0aca

As of now, I am not certain if this PG trigger is important, and if the above code change would break something.

Additionally, mergeUsers() might fail due to unic constraints. We need to figure out a good way to automatically merge such users.

[fuenkchen]

We first do #517 to get an overview of problems we have with mergeUsers mutations