Add mockup.html
Split app into tide levels/window, enable buttons

Now you can switch between tide levels and tide windows by clicking on
the links in the navbar.
Add static version of tide levels site to React
Move state TideWindowsInputBox to PilotPlannerApp
Tide-levels now populated from state, calls API

Store the time and date as strings (like with tide level) and derive the
datetime from them.
Fix collapsible navbar for mobile display

It requires bootstrap.js for the "collapsible" plugin.

Also make the navbar toggle when one of the options is clicked.