UI warnings for using scripted fields

[tbragin]

We have a warning about performance impact when a user defines a scripted field, but there are no indications in Discover, Visualize, Dashboard for when scripted fields are used. We should add some indicator when you are about to filter on a scripted field that it may be a potentially intensive operation.

[Bargs]

I think sorting and visualizing on scripted fields would also be expensive operations. It would be good to get a recommendation from the design team for representing these warnings consistently anywhere they're applicable.

There's an existing warning in a tool tip on the visualize button in Discover, which might serve as a starting point.