add --deploy option to rebuildstaging

runs preindex and deploy to staging if rebuildstaging goes through

I think people are really gonna love this! To do the whole process of rebuilding and deploying, you can now just run:

```
scripts/rebuildstaging staging.yaml --deploy
```

Features:

- uses the `--linewise` option of fab, which makes **whole lines** (!) get printed instead of one byte
at a time—which prevents the garbled text caused by parallel commands
- automatically runs the preindex
- passes `y\ny\n` into fab deploy so you don't have to type it yourself
- if any command fails, none of the other ones get executed (bash `&&`)