Add location marker moves when is moved

[ezwelty]

I don't remember this behavior when we first worked out the add-location behavior – I wonder whether it snuck in since.

Steps to reproduce (desktop only):

• Begin adding a location
• Pan the map
• → The add-location marker moves around in order to stay near the center of the map and not go out of bounds

Expected:

• The add-location marker behaves the same as the edit-location marker and stays in place unless it is dragged.