Add robot-to-operator audio streaming

[hello-amal]

Description

This PR adds robot-to-operator audio streaming to the web app. Because browsers block auto-playing media before users have interacted with the page, the audio starts muted by default. Operators always have the option, available in the header, to (un-)mute audio.

Testing procedure

• [x] Pull this PR.
• [x] Launch the interface: colcon_cd stretch_web_teleop; ./launch_interface.sh
• [x] Connect to the operator interface on a computer with a speaker.
• [x] Verify that initially, no audio is coming through.
• [x] Un-mute the audio, and verify that audio comes through.
• [x] Mute the audio, and verify it stops again.
• [x] After ~5 secs, un-mute the audio. Speak near the mic and verify there is little lag.
• [x] Repeat the above tests on the following systems:
  • [x] Stretch 3
  • [x] Stretch 2

Before opening a pull request

From the top-level of this repository, run:

• [ ] pre-commit run --all-files

To merge

• [ ] Squash & Merge

[hello-amal]

Tested on stretch 3030 and 3001, both pass the above test cases.