Use `/modules` for common modules only

[dariok]

With the move towards separate renderers (in /renderers), we now have a clean separation between functions

• that are only used for displaying views,
• those that are only used for the REST endpoints (currently /rest2),
• and those that are imported by both

Make sure that everything that is specific to only renderers or REST is kept in modules in their specific directories and only have functions imported by both in /modules.