Move mjpeg-player from av to vidgrind

[Trek Hopton]

Currently the js mjpeg-player resides in the av repo. under av/cmd/mjpeg-player. Since we are integrating it into vidgrind, it makes more sense to have it in the vidgrind repo.

Its new location should be vidgrind/player, the name mjpeg-player doesn’t allow for future expansion into playback of other media types.

[Trek Hopton]

Resolved in PR #373