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.
Use generic instance overwrite for adhocracy.hide_individual_votes
Allow instance overwrite 'adhocracy.listings.instance_proposal.sorting'
Allow instance overwrite 'adhocracy.comment_wording'
fixup 42d1ce67b7207414eca8dc5c58ab3807ba0afd04
Merge pull request #915 from nidico/feature-generic-instance-options

Generic instance setting overwrites