Put variables that can contain arbitrary strings into a block scalar. This way,
no escaping of quotes or backticks is needed.

This fixes some bugs. For example, putting a backtick into the variable
`matrix_synapse_macaroon_secret_key` previously led to an error when starting
`matrix-synapse.service`, so the service couldn't start. The error message was:

```
[...]
[...] docker[19227]: yaml.scanner.ScannerError: while scanning a double-quoted scalar
[...] docker[19227]:   in /data/homeserver.yaml, line 451, column 29
[...] docker[19227]: found unknown escape character '`'
[...] docker[19227]:   in /data/homeserver.yaml, line 451, column 37
[...] docker[19227]: Traceback (most recent call last):
[...] docker[19227]:   File /start.py, line 66, in <module>
[...]
```