Integrate attachments into pages

[frcroth]

Make it possible to attach documents to minutes and information pages.

[frcroth]

This is in my opinion the only (non-UI) item that is really holding up the launch. The migration is also ready apart from this and localization. I have tried to implement attachments multiple times but have not been successful. Here were my ideas:

• Create a new model that extends AbstractDocument. Then every page has an abitrary amount of attachments. Since attachments would need to be stored on pages, it would make sense to also create and manage them in the page edit view. I did not manage to create a widget that allows for the creation and deletion of documents.
• Create a new model that extends AbstractDocument. Also, this model includes a group field that determines which group "owns" the attachment. Only users within that group should be able to see it when selecting attachments in the page edit. I did not yet find out how to limit the selection in a field panel, whoever that should be doable. This would mean attachments would need to be created in a seperate admin view (like documents right now) that also allows for the assignment of documents. Perhaps this is the easiest way.
• Use documents as attachments. To have some rights management, use the collections. Since collections do not really interact with groups, some connection would need to be created.

Displaying the attachments on the page and creating download links was never a problem. Problems mostly arose from the rights management.