Don't seed the release date if it's before the source service existed

[Aerozol]

The functionality to filter out release dates if it's before the source service existed.
Harmony internally already knows the start dates of each service.

Some notes from kellnerd:

• Harmony should not censor source data, the doubtful dates should still be displayed but not seeded. Otherwise I fear that people will either think that Harmony failed to get a date or just copy the doubtful date from somewhere else.
• The current date objects could be extended with a new optional property which explains if/why a date is probably incorrect and will not be used. Maybe we even need an additional “quality” flag because dates from some providers can’t be relied on in general (like iTunes always using the original release date, which is only correct if the digital release happens to be the first release).
• Placeholder dates like January 1st should probably be auto-corrected while still being transparent about this behavior. Not sure if/how this should be combined with the new “explanation” property, maybe this should only happen at the merge stage when the placeholder date is the only remaining plausible date candidate.