It is now possible import and export decks. Decks can be imported from
OPML files. For each import a new deck is created. If the OPML file
contains nested `outline` tags, we will create one column for each
parent `outline` tag. Otherwise we will create a column named `Unknown`
and all sources to this column. When a deck is exported we will create
one `outline` tag for each column with it's sources as siblings.