fixup 414431e for old browsers without history api
Remove unneeded template variable

Fixup f3e57abf0c0b9c7b3b611ea79090a6e3f9de3ad5
Fix badge impact migration if badges are present

If badges are already present in the system, their impact value must be set to something other than NULL. (default=0 is misleading, since it just applies to newly created badges, see http://stackoverflow.com/q/16097149/35070).
Ignore minified stylesheets
Use adhocracy.config in lib.base
Allow to customize description in html head
Allow to freeze single proposals (fixes #559)

This allows to mark proposals as frozen for users with a newley
introduced `proposal.freeze` permission, which is by default assigned to
instance admins.

If a proposal is frozen, it isn't possible

* to edit the proposal (apart from instance admins),
* to comment the proposal (apart from instance admins),
* to rate / to delegate the proposal.

It's also not possible to rate selections, while it is still possible to
edit selections and to comment on selections. These restrictions will be
implemented after merging the sectionpages branch.