Move non-essential parts to separate package

[Stiivi]

The following parts should be moved into separate packages:

• Mongo backend
• GA backend
• Mixpanel backend
• Visualizer
• Modeler

Other tasks:

• check dependencies in the separated packages
• remove dependencies in cubes that were backend specific

[Stiivi]

Backends are now removed in their respective projects:

• MongoDB
• Google Analytics
• Mixpanel

Clean-up was done in 7508f32 (read the commit message for more details about promoting SQL to the top level module).