Allow to set how much data is "quite a lot"

[RicoElectrico]

For decent PCs 2 or even 10 MB of data on the map is not going to cause serious problems, there may be some lag, but no freezing.

[tyrasd]

This is true, but it also depends very much on what kind of data is returned: While 10MB of large, complex polygons is typically no problem at all, but 10MB of points may be an issue (because of the large number of created SVG/DOM elements).

But, I admit that a customizable limit does indeed make sense.

[DaveF63]

Are these requests related to #269?

[tyrasd]

suggestion from @bjohas: make this configurable on a per-query basis: e.g. something like {{largeDataWarningLimit:10MB}}