Narratives / Threads / Trails

It would be nice to have a way to build a sequence of annotations, and show them together. Would be especially useful together with #384.

The goal is to have a way to present a sequence of thoughts, quotes, whatever, that together tell a story, or illustrate a train of thought, or something like that.

Could also be used for presenting a reading list, or a manually created news feed.

This is somewhat similar to firehose ( #356 ) with configured filters, but not the same, because it's not generated on request, but stored in the DB, and we actually have to have a UI for building it.

For showing a narrative: we could have linkable narrative pages very similar to linkable annotations, with the following differences:
- multiple annotations are shown, in the pre-determined order, not only one
- replies are optional (user should be able to toggle them on and off)
- maybe there could be really short text sections for connecting the annotations. (Showing text that is stored as part of the narrative.)

For building a narrative: we need a UI that supports drag-and-drop building and reordering. (We should probably show only a shortened form of the annotation, not the full text of body/quote). We also need some feature to edit the text segments connecting the annotations (if any).