New Pokéstop marker flies off screen

### Describe the bug
The purple new Pokéstop marker flies off screen when dragged out of bounds.

### To Reproduce
**Steps to reproduce the behavior:**
**1.** Right click to place a new Pokéstop marker
**2.** Click it and drag it towards the edge of the map boundaries
**3.** My people need me

### Expected behavior
New Pokéstop marker should stay within bounds or be removed entirely if that's not possible.

### Screenshots
If applicable, add screenshots to help explain your problem.

### Platform
**- ResearchMap version:** development and master branch
**- Device:** Desktop
**- OS:** Windows 10
**- Browser:** Chrome

No source file, test, or entry point is named in the issue. First reproduce the drag near the map boundary in the development or master build, then trace the new Pokéstop marker's drag handling; done means the marker remains within bounds or is removed when it cannot.

Written by the indexing model from the issue text.