UI to create / edit cklists

[romanc]

Support for editing / creating new cklists is not complete yet. In particular, we don't have UI to create / edit the cklist document.

Together with #12 and #13 this brings the question if we want to allow editing of cklists: In particular, if cklists change over time, we'd need special treatment for inspecting completed cklists that changed meanwhile. To keep things simple, we might not allow to change the cklist document. Changes to title / description of the cklist should be okay.

One argument for allowing to edit cklist documents is that it might be annoying to be unable to fix say typos in cklist steps.

[romanc]

For the first version, let's just not allow changing checklists. That will also simplify and streamline the MVP's UI.