frontend: add debouncing to recording functions

### Current behaviour

You can click twice fast when starting a recording, and the recording stops. A user might still think it started, and miss out on recording a session as a result.

### Expected or desired behaviour

Add some debouncing to disallow very short recording start/stop inputs from the user.

### Prerequisites

- [x] I have checked to make sure that a similar request has not already been filed or fixed.

Start by locating the frontend recording start and stop handlers, then reproduce the rapid double-click behavior described in the issue. Done means very short successive recording inputs are debounced so a fast second click cannot unexpectedly stop a newly started recording.

Written by the indexing model from the issue text.