feature: add embedded 4four.io metronome above video component

[ryancraigmartin]

Summary

I've been using Fretonator a lot lately and was pleased to see that it was an Angular app! I'd like to bring this simple, hidden metronome embed up for consideration as an add-on to the existing site. I loved the simple look of the Learn about the 7 patterns button and thought using it might be a nice way to break the sections up. I know the light theme of the metronome embed can be a bit abrasive on the dark theme, but open to suggestions!

• adds metronome iframe wrapper from 4four.io and a show/hide button styled like the Learn about the 7 patterns button.

Screenshots: