New ActionIfExists: Append or merge

[dpriskorn]

While Working on https://www.wikidata.org/wiki/Wikidata:Tools/WikidataEurLexScraper I would like to merge the references of official name statements that are identical

So when I add a claim with the same official name and another reference I would like WBI to merge so that the statement has two references.
If he references are exactly the same they should be merged.

If there are different qualifiers we should raise a QualifierMergeConflict error

[dpriskorn]

duplicate of #607