Found a few instances, where celery intermittently complained about a
misconfigured redis instance and exited.
> redis.exceptions.ResponseError: MISCONF Redis is configured to save RDB
> snapshots, but it's currently unable to persist to disk. Commands that
> may modify the data set are disabled, because this instance is
> configured to report errors during writes if RDB snapshotting fails
> (stop-writes-on-bgsave-error option). Please check the Redis logs for
> details about the RDB error.
The state directory contains static files that need to be accessible by
a webserver, but homeMode defaults to 0750 and switching the generation
will always force the homeMode, thereby breaking access to the assets.
Instead, fully rely on systemd to provide the StateDirectory with the
correct mode.
Previously, pdftk (part of the ticket, badge, ... generation pipeline)
would fail with:
```
Error occurred during initialization of VM
Failed to mark memory page as executable - check if grsecurity/PaX is enabled
```
Thise caused pdf generation to fail.
Since pdftk is a java application and, according to systemd.exec(5),
> Note that [MemoryDenyWriteExecute=] is incompatible with programs and
> libraries that generate program code dynamically at runtime, including
> JIT execution engines, executable stacks, and code "trampoline" featu
> re of various C compilers.
Disabling `MemoryDenyWriteExecute=` fixes it.
