Consider updating MAR_CHANNEL_ID for nightly build (and maybe alpha too)

In browser/confvars.sh (from tor-browser.git) we currently set MAR_CHANNEL_ID to torbrowser-torproject-release in all cases.

I see that Mozilla is using a different MAR_CHANNEL_ID for each of their channel (previously by updating browser/confvars.sh, and now by setting it from taskcluster: https://hg.mozilla.org/releases/mozilla-release/rev/66f52bda7e14e26235bd0a43bb68ad11775046e4).

So I am wondering if we should do the same, and use a different MAR_CHANNEL_ID for nightly, and maybe for the alpha too.

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information