Add video preview and countdown to RecordRTC workflow

The current workflow and UX for recording audio and video could be improved.  When the tool opens, the user is not given enough information about what to expect.  There is just a single, very wide, button that says "Start recording" with no other information.  Clicking it immediately begins recording.  While that is a very literal result of clicking the button, it's not the clearest approach from a UX perspective.  Ideally, upon activating the RecordRTC tool, the user should be presented with some sort of video preview above the "Start recording" button. After clicking the button, there should be a brief 3-2-1 countdown.  Only then should recording actually begin.  These adjustments, especially the preview, would put Moodle's tool in line with other online recording experiences.

This is an issue in 3.5.x but the current documentation does not seem to indicate that it's been part of later versions.