Allow modifying tide level input box

Completely unconstrained, but demonstrating the up/down flow of
information in React.
Validate that inputted number is a float and round

We limit the precision of the input box to 1 decimal place.

Note: Factor out the magic number (1) with in an idiomatic Javascript
way.
Fire off an AJAX request when the state changes

When a new value is entered into the input box, fire off an AJAX request
to get the tide windows for given location & tide level.

Display these in the user's local timezone.

Note that it won't currently abort previous AJAX requests.