Flush on set()
Improve in_notification_window

The check for occurrence in the past takes only date into account. Now
the time-aware filter in done in the task. Also, notification before
days defaults to 1, since 0 causes skipping notifications of occurrences
happening before notification hour.
Add tests for is_in_notification_window
Add freeze_time fixture for tests

Freezes the time returned by datetime.now etc. and also by sqlalchemy's
func.now()
Add tests for is_in_digest_window
Add test for round_up_month
Include plugin engine
Add IndicoPlugin class, provide CLI hooks
Let plugins specify blueprints
Use entry point for plugins
Plugin system refactoring
Support plugin assets
Skip compat blueprint if old urls are disabled
Use nicer name for plugin asset files
Improve admin access check

Don't fail if checkParams hasn't been called first...
Basic admin interface for the new plugins

Only a list so far and a plugin detail page showing the description...
Add WPJinjaMixin to use jinja nicely in WPs
Add plugin settings
Allow redirecting using flask.redirect()
Remove `indico db init`

We always have an alembic env so there's no point in exposing that
command.