Auto-generated translation credits TMs aren't contributions.

Bug 128324 has been fixed by creating special TranslationMessages for translation credits messages.

The creation of these messages is attributed to the POFile's last translator if set; otherwise to its owner, which more or less defaults to (but isn't always) rosetta_experts. A POFileTranslator record is created for the contribution. This causes some wtfs:

1. rosetta_experts is listed in translation credits messages.

2. Spurious contributions might be logged for POFiles you caused to be created but never worked on.

Also, POFile.owner is practically unused otherwise and the semantics for setting it are not terribly meaningful. Changes there (e.g. the fix for bug 443129) affect the translation-credits tests unnecessarily.

It makes more sense to attribute these special messages to rosetta_expert consistently, and to exclude rosetta_experts from contributor lists.

Note that this is not urgent enough to warrant a data conversion—or interrupting the ongoing fixup of existing translation-credits messages.