Fix a bug in airstrip/base filtering

If an airstrip is selected, base filtering should be ignored.
Align the add/remove URL with the filter URL

I'm still not sure whether using checkouts/ and checkouts/edit/ makes more
sense than checkouts/ and update/, but I'd like to give it a try.
Integrate flash messages to update the user
Add IDs to some base template elements for styling
Remove old version of aircraft_type form widget
Add colors and improve form layout

Mostly affects the add/remove page, but they all benefit from the header update
Complete the refactor of 'update' to 'edit'