Introduce config helper functions

Add get, get_bool, get_int, get_list helper functions to
`adhocracy.config` and reexport `adhocracy.config` to
`adhocracy.lib.helpers` for consumption in templates.

This reduces some imports on the one hand, but more importantly allows
to define default values in one defined place only and using python
types, which is more clear and saves some type conversions.
Make use of new config helper functions