Refactor creation of map process

[JannikStreek]

Change the way the create button on the start page works. Currently maps are created when accessing GET /mapwhich 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