Reduce bundle size

With the introduction of date validation, house-rules gained weight dramatically (due to Moment.js).

Can we do something about this?

- How about replacing moment with something lightweight, such as [date-fns](https://github.com/date-fns/date-fns) or [Luxon](http://moment.github.io/luxon/)?
- Is date-specific dependency even necessary? It looks like validator already implements `isAfter` and `isBefore` methods.
- Can we make date validation opt-in?

Start by locating the date-validation code and the Moment.js dependency in house-rules, then inspect how the bundle is produced and measured. Compare the proposed alternatives and whether validator's isAfter/isBefore methods cover the current behavior. Done means agreeing on a concrete approach that reduces the bundle while preserving date validation, including whether it should be opt-in.

Written by the indexing model from the issue text.