Change pantheon pull to use terminus backup:get

[rfay]

The Problem/Issue/Bug:

The previous (drush rsync) technique for pulling a backup from Pantheon was just way too tedious, and could even fail due to connection timeouts

How this PR Solves The Problem:

Use terminus backup:get. command and require that a backup already exists.

Manual Testing Instructions:

Use the "pull" sections of the new strategy with your site and see if it works.

[amitaibu]

@rfay Worked great! (I only wish there was a progress report in terminus for the files get -which in my case took quite a long time)

Two suggestions:

1. Since db_push_command and files_push_command are dangerous, how about commenting them out? Let's force user for one more step, before they accidentally destroy their env.
2. Since files_push_command is now the only command using drush directly, I think we can remove the terminus aliases 2>/dev/null from the auth_command and move it to be under files_push_command. Like that we'd save a bit of time on each authentication.

[rfay]

I agree with you on both.

It was actually DDEV (the company) that kept pushing for "push" for integration with DDEV-Live, and I probably wouldn't have done it anyway.

The time for terminus aliases is irrelevant compared to the time for pull though.