Make user_force_term more flexible with a settings table.

I'd like to start adding some flexibility to this module by creating a flexible settings table i.e. 'ft_settings'

Attached is the proposed scheme.

Fields overview:

• level: Defines the hierarchy of default taxonomies. i.e. user, role, node type.
• taxonomy_id: the id of the selected taxonomy for that level.
• visibility: Determines whether the terms should be shown in the node/add form or not. Defaults to 0 or bool false.
• enforce: Determines whether this level will require a tid or not. This is built for backwards compatibility with the existing version.
• weight: Determines which value should be considered first in case all three levels are set.