Found in production. Yes really. Due to some borked LXC integration, my
snikket host reports
```
MemTotal: 9007199254740991 kB
MemFree: 9007199254690591 kB
MemAvailable: 9007199254690591 kB
```
That is more than 1024 TiB, so it tries to go further up in the scale,
which then causes a Guru Meditation because of the uncaught IndexError.
Previously, some kinds of errors would throw nice and fun cascades
of exceptions.
We now have a nice, clean error page for 500 and 503 (backend
connectivity) errors which includes minimal debugging information
for productive setups and a traceback for development setups.
In any case, the full exception is logged to the log with an error
ID which is printed on the error page.
- Avoid fighting import cycles using a factory function
- Collapse useless subpackages into simple modules
- Move flask plugins / infrastructure in own module
- Refactor how blueprints are used to localize information about
URL routing to app factory
