Allow generic instance setting overwrite
Allow generic instance setting overwrite

All settings can now be overwritten on an instance base by

1. activating allow_overwrite on a setting, e.g.

    adhocracy.comment_wording.allow_overwrite = true

2. using it, e.g.

    adhocracy.comment_wording.aalen = true

For some settings, which commonly use instance overwrites, this will be
enabled in adhocracy.config.DEFAULTS.

phihag/adhocracy,liqd/adhocracy,alkadis/vcv,phihag/adhocracy,liqd/adhocracy,phihag/adhocracy,alkadis/vcv,DanielNeugebauer/adhocracy,DanielNeugebauer/adhocracy,phihag/adhocracy,alkadis/vcv,phihag/adhocracy,DanielNeugebauer/adhocracy,DanielNeugebauer/adhocracy,liqd/adhocracy,alkadis/vcv,alkadis/vcv,liqd/adhocracy,DanielNeugebauer/adhocracy