Feature/scenario create

See commit comments for details, probably best to view diff just for https://github.com/WorldBank-Transport/open-transit-indicators/commit/4d5f26094528be672e93e0c0cb9cd5ab5c6dc6b1 since that's where all the meat is.

Soliciting feedback on the new model structure, should be a bit more flexible and isolated.
Everything is still sort of on the same level even though they have their own folders now; would this make things cleaner?
```
/list/list-controller.js
/new/new-controller.js
/new-success/new-success-controller.js
/routes/routes-controller.js
/routes/edit/route-edit-controller.js
/routes/stops/route-stops-controller.js
/routes/shapes/route-shapes-controller.js
etc.
```

Or another possibility (more radical):
```
/scenarios/list/controller.js
/scenarios/new/controller.js
/scenarios/new/success-controller.js
/scenarios/routes/controller.js
/scenarios/routes/edit/controller.js
/scenarios/routes/edit/done-controller.js
/scenarios/routes/edit/stops/controller.js
/scenarios/routes/edit/shapes/controller.js
etc.
```