Generic instance setting overwrites

In the recent development period, we have repeatedly introduced new settings on instance level as installations option in the form, in a lightweight manner (without migrations), e.g.:

    adhocracy.comment_wording.instance_name = foo

This commit provides a generic way of overwriting installation settings on an instance base. In order to allow a setting to be overwritten as above, it has to be declared, e.g.:

    adhocracy.comment_wording.allow_overwrite = true

This PR makes use of this feature in three - somewhat arbitrary - commits:

* The already existing `adhocracy.hide_individual_votes` instance setting is ported to this system.
* `adhocracy.listings.instance_proposal.sorting` can be overwritten by default.
* `adhocracy.comment_wording` can be overwritten by default.

More can be ported / enabled later.