Getting locked out while flying through strings

[mathjazz]

We've received reports from users getting locked out of Ponton when they've been “flying through” strings in the translate view.

This happens because:

• We have rate limiting set in Fastly: If 500 counting signals in 1 min, take action for 10 min
• Openning each string triggers multiple network requests

Since we want to keep the rate limiting around, we should reduce the number of requests we make in such scenarios by:

• detecting rapid key navigation and delay reqeusts by a few 100 ms.
• prefetching next/previous string data in background (even for MT), which would improve the UX in general.