Refactor creation of map process

Change the way the create button on the start page works. Currently maps are created when accessing GET `/map`which is not really a proper REST style way of object creation.

Idea:

1. Async request, create new map on server
2. Wait for answer, use id to proceed to map URL.
3. Take care of 1) using hotkey to create a map, 2) Creating a map from start screen, 3) Opening an existing map by link

Trace the start-page create button, the current GET /map behavior, the hotkey path, and opening an existing map by link. Verify how creation requests and map IDs flow through these entry points; done means creation is asynchronous, navigation uses the returned ID, all three scenarios still work, and GET /map no longer creates a map.

Written by the indexing model from the issue text.