Add release date to rbm.conf

I think we can add some release date in rbm.conf, next to torbrowser_version, to be updated when preparing a new release.

I think it can then be used in the following places:

as timestamp on the files from the bundles (currently it's set to 2000-01-01, which makes some users confused according to #11506 (closed)).
as MOZ_BUILD_DATE for building firefox/geckoview. Currently we have some script that generate a fake date based on version number.
as release date for the debian package (#41083 (closed)). Currently we use the date from the last firefox commit, but it's not guaranted to be always a meaningful date.

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