Implement FSM for flows

[henhuy]

Implemented flows having multiple states and transition between the states.
Visit http://0.0.0.0:8000/roof/ to start flow and see simple transitions when changing roof type.

For orientation look at the example flow RoofFlow
and related template roof.html.
Note that you have to add div placeholders for each state and that you have to add rendering of the partials using safe within the placeholders (this is in case the whole page gets rendered, not only next state HTML via HTMX).