Bug: Prevent import of worlds if server is running

If you import a world while the server is running and set to that same world, you’ll have the problem that the new imported data will be ignored and the next operation you do in the world (such as moving a token) will cause the database to be overwritten with the cached version that the server had kept in memory. This can cause a corruption or invalid state of a world.

Preventing import of worlds without prior stopping the server can therefore lead to an undefined behavior.