Update script

[d-buchmann]

Update script that WorksForMe*
Might be a starting point for someone to develop a more useful general update solution (#720).

The constants have to be adjusted to your local (bare metal) environment.
I don't have enough experience with docker to approach that.

The (guided) update process goes through ~4 stages:
0: check for new release
1: put current site into maintenance mode (requires an extra vhost to be set up...meh)
2: backup partdb database (only mysql at the moment...meh); git pull
3: Build according to the docs, transfer required files to testing location (yet another vhost...)
4: Transfer testing site to live enviroment, end maintenance mode

The process can be interrupted at any stage, by errors or by user interaction.
Resumption through current stage stored in a file with the name of $FILE. While I'm writing this, I think it's easier to use an env var.
Also, media files are at least copied twice, so performance gain is possible.