Web UI: disable syntax highlighting for massive JSON documents

[mglukhovsky]

We disable syntax highlighting for a large number of rows, but parsing and highlighting a few massive documents can sometimes crash the tab in the browser when using the Data Explorer.

We should profile and determine a good size limitation, and turn off highlighting for very large documents.

[danielmewes]

There are some reports that it's already getting turned off for even a single large document. We need to find out what our actual logic currently is.
It's also possible that the browser freezing is due to something other than syntax highlighting.