Implement golden month of beer

As requested by the board, a so called "Golden Month of Beer" should be introduced where all members get two free beers per day.

This merge request introduces the option to specify a start and end date during which all amiv members get two free beers (still one coffee only).

This MR can be merged any time. The time interval during which the feature is enabled must be specified in the configuration file. See the example config file at instance/config.example.py.