Link old deb.torproject.org experimental repos to stable

On tor-dev, Iain suggests:

As an alternative strategy, symbolic links for old alpha repositories
point to the current stable repository. If you're not updating your
sources.list you end up on the stable branch. I think this would mean
"less surprises" than jumping to a new alpha branch.

https://lists.torproject.org/pipermail/tor-dev/2018-July/013251.html

Which is pretty much what weasel said in #14997 (closed):

Running the current alpha should always be a deliberate decision.

If you can't be bothered to change your sources.list once or twice a year, then you probably should be running stable.

https://trac.torproject.org/projects/tor/ticket/14997#comment:3

For example, since 0.3.3 is now stable, we could link:

tor-experimental-0.3.3.x-jessie -> jessie

And when 0.3.4 goes stable, we would create:

tor-experimental-0.3.4.x-jessie -> jessie
tor-experimental-0.3.5.x-jessie

But what do we do with tor-experimental-0.3.3.x-jessie -> jessie when 0.3.4 is stable? Delete it? Then operators don't get updates. Keep it? Then operators get 0.3.4 when they're expecting 0.3.3.