new feature: create snapshot of doku instance

[ax-sc]

The snapshot should include documents (as a doku-object, not as a PDF), resources, stylesheets, templates and snippets. Being able to bulk-download all PDFs is nice, but doesn´t help us in the case we have to restore our doku instance as every document would need to be created from scratch. It would be better to have a feature to create a snapshot of all relevant information and also be able to inject this to a running doku instance. The possibility to setup a scheduled task to create snapshots or at least remind the user to download them would be a nice add-on.