Store session data in database

## Changes

### buildout_common.cfg
    
* Not intended remove of whitespaces at lineends
* Added option `beaker.db.url` to tell beaker what database it should store ses
    
### buildout_development_mysql.cfg
    
* Added option `beaker.db.url` to tell beaker what database it should store ses
    
### buildout_development_postgres.cfg
    
* Added option `beaker.db.url` to tell beaker what database it should store ses
   
### etc/adhocracy.ini.in
    
* Set the options to configure beaker to use a database for the sessions
* Unset the beaker option that configures session data dir
* Moved the unset option for backup 5 lines up

## Fix

* Fixes [hhucn/adhocracy.hhu_theme/issues/#59](https://github.com/hhucn/adhocracy.hhu_theme/issues/59)