allow to re-order series elements by drag-and-drop

[maxlath]

see https://github.com/inventaire/inventaire/issues/89#issuecomment-305866181

clues on the implementation:

• this probably requires a dedicated view, that could be called from serie_layout (that layout)
• probably requires to rely on a drag-and-drop library like dragula or draggable
• could, at first at least, be made available only for the most favorable cases:
  • all series elements that have a series ordinal (wdt:P1545) defined should be positive integer strings (match ^\d+$). This should always be the case for inventaire entities as it is the only format accepted server side, but that is far from being the only format on Wikidata (example)
• should probably allow to not have all elements sorted. In the case of our example, there is for instance this composite editions that shouldn't be part of the re-ordering. More genearlly, it could allow to do a partial re-ordering without having to deal with the whole series.
• how to handle missing elements? there could be a system of placeholders for missing numbers: if a series has a work 1 and 3, the view could insert a "create missing work" between the 1 and 3 to easily add the 2

[maxlath]

work in progress: series cleanup layout

branch: serie-cleaner

still missing:

• drag and drop, not that easy to implement, is that really needed?

[maxlath]

other drag&drop lib: interactjs