Should be able to cancel large databrowser downloads

[gravitystorm]

When the warning appears, the user might decide it's not worth waiting, and want to cancel the display. An adjacent cancel link would let them do this.

[tomhughes]

I'd suggest the normal close icon in the top right right for that.

[tomhughes]

I've pushed a change to add a close button. but I'm not sure it actually works that well as an idea because as soon as you move the map at all it is likely to popup again.