Rework date parsing helpers

[kellnerd]

Harmony has a few utility functions to parse dates into the PartialDate format.
parseISODateTime currently only expects the YYYY-MM-DDTHH:mm:ss.sssZ format according to its description, but we realized that it would also work for mora and SoundCloud.

We should write test cases for the date helpers and then try to combine them into fewer, less specific functions. The current parseISODateTime can probably handle more formats with the same function if I add a few safeguards on top of the notoriously surprising JS Date parser.