This PR adds a button to the Stairs page that allows adding multiple parallel stairs at once (i.e. a staircase):

Even strange staircase shapes are supported (the stairs are created all parallel to the first edge in the polygon):

I am not completely sure about some of the changes I made, and I am happy to improve on them if I receive feedback:

• When saving N stairs, N requests are made to the server using the existing /editor/spaces/{SPACE}/stairs/create endpoint.
• When I first implemented this feature, when sending N requests all at once, most of them would get lost without giving any error, apparently due to the fact that they all created different changesets concurrently which then disappeared. Now I made it so that a request for a single stair is sent first, so that a changeset is instantiated, and then all other N-1 stair creation requests are sent, which end up in the correct changeset. Is it expected that there are such concurrency issues?
• I added a new HTML template for the staircase editing page. Since that page needed mostly the same runtime checks and template variables as the stair editing page (i.e. editor/edit.html rendered by edit() with model=apps.get_model('mapdata', 'Stair')), I added a special case in edit() that renders a different HTML page if the staircase endpoint is provided.

Let me know your thoughts.

_{Note: this code was initially written during a Hackathon together with @dennisorlando @Degra02 @Usioumeo. The hackathon was organized by NOI, which also hosts SFScon, which uses c3nav for navigation during the conference.}