Merge branch 'v1.9'

Conflicts:
	indico/core/db/sqlalchemy/util/management.py
	indico/modules/rb/models/reservation_occurrences.py
	indico/modules/rb/models/reservations.py
	indico/modules/rb/models/rooms.py
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.
Load models from plugins
Typos
Delete request memoization cache on commit

Otherwise e.g. Location.default_location will return an expired object
after committing
Add plugin db migrations (`indico plugindb`)
Document the new plugin system