Clean up the requests array whenever user hits "Clear"

[kdzwinel]

Since frontend doesn't store response bodies, but requests them every time user opens "Preview" tab, we have to store them on the backend. This can quickly fill up the memory, so we should clean that array whenever user clicks "Clean" on the UI. Unfortunately, clicking that button doesn't send any message to the frontend, so right now there is no way to figure out if we can clean the requests array.