File storage

[jeriox]

As a planner, I expect to have a view where I can upload new files and delete previously uploaded files. As a user, I expect to have a view where I can view a list of uploaded files and download them.

Optionally: As a planner, I want to attach a file from the file storage to an event. As a user, I expect the event detail page to list the linked files and allow me to download them.