Improvements to date entry

[bryanbraun]

Things I could do, in rough order of value:

• Save a user's birthday in local storage and prepopulate when revisiting the page. [fixed in https://github.com/Improvement/local storage #6]
• If a user clicks the dropdown to switch between weeks/months/years, check to see if a date has already been chosen and pass that date to the next page, using either localstorage or query parameters. [fixed in https://github.com/Improvement/local storage #6]
• Improve validation (no days above 31, or even month-specific validation).
• Improve arrow-key incrementing/decrementing of days (by preventing users from choosing invalid days). A truly accurate version (with month-specific validation) could be pretty complex.

Even better: Switch to an <input type="date"> field

• Browser support is good now
• This would prevent invalid entries and improve UX and accessibility on both mobile and desktop.