[Sanitization] Fine-tune input parsers (#192)

Add tests for various input parsers. Fixed the following bugs:

* `searchCardsParamsValidator.cardIDs` checks for csv mongo IDs.
* `addCardParamsValidator.tags` and `partialCardValidator.tags` check for csv.
* `partialCardValidator.id` is required.
* `partialCardValidator.urgency` checks for finite non-negative numbers.
* `partialCardValidator` drops `metadataIndex` and `createdById` as these are read-only.
* `partialCardValidator.lastReviewed` parses the value into a `Date` object.
* `partialCardValidator` checks that `numChildren`, `numTimesMarkedAsDuplicate`, and `numTimesMarkedForReview` are non-negative finite integers.
* `userSettingsParamsValidator.cardsAreByDefaultPrivate` checks for the `on` literal.
* `userSettingsParamsValidator.dailyTarget` is now optional.
* User names must be alphanumeric and at least 3 chars long.
* Passwords must be at least 8 chars long.