allow proposal pagination

From https://github.com/liqd/adhocracy/pull/904#issuecomment-44430541:

> We just discovered that the individual links aren't permanent due to pagination. If an item is linked to from a category page and the item's category changes, the link also breaks.

This pull requests allows to disable pagination on proposal pagers if people need to be able to link to pager rows.

For me there was an issue with the `default=True` part. The new instance option was False by default. I guess this was caused by migration issues with sqlite. Hopefully we can clarify this.