Views: allow setting the "Items to display" value to "unlimited".

This is basically the same as #1459 but for Views instead the fields.

Here's what the UI looks like now:

I think we should apply the same limited/unlimited drop-down menu pattern here as well, but with the default being set to "limited" and "10" (so that we don't change the current default). The end goal is to allow users to select "unlimited".