[DateRangeInput] Show formatted min/max dates on focus when inputs empty

[cmslewis]

Addresses #805

Checklist

• Include tests

Changes proposed in this pull request:

• Show formatted min/max dates on focus in an empty start/end input

Reviewers should focus on:

• Things break if min/max === null, but that's already not really a supported case. Can address that more generally in a future PR if we want.
• See comments about caching the formatted min/max date strings. If we don't cache the formatted min/max date strings in this.state, then we'll have to cast those vanilla JS dates to moments on every render just to achieve properly formatted placeholder dates, which is exceedingly wasteful.

Screenshot

[blueprint-bot]

Little cleanups

Preview: docs
Coverage: core | datetime

[blueprint-bot]

Resolve conflicts

Preview: docs
Coverage: core | datetime