fix(datepicker): correct minValue and maxValue displays and allow same date selection within Range

[austingreendev]

Description

While making some additional datepicker examples I found a few issues that I have resolved in this PR.

• minValue and maxValue displays were off by a single day due to us comparing date values without finding a common day baseline
• DatepickerRange usages were unable to have a start and end date that were the same value

Checklist

• 👌 design updates are Garden Designer approved (add the
  designer as a reviewer)
• 💅 view component styling is based on a Garden CSS
  component
• 🌐 Styleguidist demo is up-to-date (yarn start)
• ⬅️ renders as expected with reversed (RTL) direction
• ♿ analyzed via axe and evaluated using VoiceOver
• 💂‍♂️ includes new unit tests
• 📝 tested in Chrome, Firefox, Safari, Edge, and IE11

[jzempel]

It would be good if the range example could be tweaked under this PR. The current grid layout isn't working very nicely.

Instead, we'd want those fields to display at a fixed 280px width directly above the corresponding begin/end months.