Extract saving

Also helpful for #193 

Extracts handling of persisters to an own `saver` service. It's not doing much right now, but this will be the place where we do more elaborate things eventually.

Also adds a `saver` directive, which is right now e.g. used in the `navbar1.html` template. The clickable element to save is only shown when persisters are present - otherwise we don't need it, as we cannot save anything anyway.