Provide undo-operation on delete

[stefan-niedermann]

When one deletes a board or a card, there should be an undo operation available for 4 seconds, so if a user deletes something by accident, it can be easily restored with a single click.

[juliusknorr]

Makes sense. Also related to #70

[juliusknorr]

Maybe as a quick fix we can just delay the deletion for 10 sec, to provide that kind of undo functionality. A good starter issue I think.

[juliusknorr]

Basic implementation is in #113 but we should just mark the board as deleted in the database and add a cleanup job to finally remove those boards after some time. Then the user doesn't have to wait the 10 sec. until the request for deleting is sent to the server.

[juliusknorr]

implemented with #133