Allow to make email optional

This allows to make emails optional on an installation base. Making
emails optional makes the "require email validation" instance option
meaningless.
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
Merge pull request #450 from nidico/feature-make-email-optional

Allow to make email optional