Change conflicting zoom map in/out by a lot shortcuts

[quincylvania]

All major browsers use the Cmd+ and Cmd– shortcuts to zoom the page (Ctrl on non-Apple systems). This is an important accessibility feature, but iD currently overrides these shortcuts and instead zooms the map several zoom levels.

I propose changing iD's shortcuts to Alt+ and Alt–, which doesn't appear to be used yet. For consistency I'd also like to change the modifier for nudging the selected features or the map with the arrow keys by a lot from Cmd to Alt.

Changing keyboard shortcuts is never great, but I believe first-class accessibility should take priority.

Re: #7713