Refactor common logic for edit pages

Currently the edit pages all follow a very similar structure, and all have some behaviour that is not specific to the page itself. This includes:

• handling of edit mode
• handling of the question if the user is logged in (see #60 (closed))
• handling of the backup process (for a deep restoring if the user cancels the editing)
• handling of url changes once a new entity is saved (#54 (closed))

We want to collect all of this behaviour and put it into mixins (if possible), so that we can maintain those functionalities on one place and reuse it for devices, platforms, contacts & configurations (and maybe also sites later).