This changes the API of pretix.control.signals.event_dashboard_widgets and
pretix.control.signals.user_dashboard_widgets, such that the "content" key of
returned dictionaries must contain a SafeString if HTML therein is to be
rendered.
* Navigation and dashboard: Hide useless items
If a user has access to *no organizer teams*, hide a number of things
from navigation and dashboard. This happens e.g. if a user only has
permissions in scope of the pretix-resellers or pretix-scheduling
plugins.
* New mechanism
