Remove unused module/class
Code style, missing docstring
Split indico.core.db.sqlalchemy.util into modules

Also move some utils from indico.modules.rb.models.util
Move Serializer to indico.util.serializer
Remove unused function
Rename/document/move double_wraps decorator

It is incredibly useful but now it has a (slightly) better name and
proper documentation. It also is in a more appropriate place.
Move settings model into core
Use a nicer way to import all models