Set sass charset to utf-8

This makes sure this will also work on a system which doesn't use a
utf-8 default locale.

This also needs to be put on top of the main files in the theme.
use adhocracy.config in adhocracy.lib.notification
resolve circular import
Add unique (event, user) constraint to notification table

Make sure that only one notification exists in the database for each
event and user.
Make sure that notification aren't added twice
Add replay notification script

Add a script which allows to replay all events in order to create
Notification entries to the database which do not exist yet.
clearify the use of amendment/is_amendment in proposal.create
add entity_url tests
use anchor parameter instead of manually appending it
define page root
fix page entity_url
fix sectionpage comment url
use member in proposal_helper.url
amendment url in overlay
use in_overlay=False when navigationg inside an overlay
fix page delete form action
fix page edit form action
exclude hidden instances from user.instances

I would have prefered to make this optional but as instances
is a property that would be complicated
hide events from hidden instances in profile page

but only if the instance is not explicitly selected.
This is consitent with how `hidden` works everywhere else.
unfinished: block notifications from hidden instances