Merge with existing translation on review / accept

Having a job with incoming translations is no guarantee there's no conflict with what we review.
Currently, accept leads to overwriting a possible previous available translation.

If the job is old (outdated) and the current available translation is a newer, we're doing the wrong thing.
A warning should be displayed at least if a translation is already present. "You're about to update/overwrite a translation"...

If we create a job on node creation (not jet reviewed) and later create a new job on node update (e.g. via rules)... This situation is very likely:
If the new job is reviewed first (accepted) and the old later, we're in that situation.
An old job possibly should be marked as outdated.

Possibly related to #1686544: Check for source change on review and show diff