Add view for all RPs without any subgroups

[irisfofs]

Right now you can't actually just look at a chronological list of RPs. There's the canon/noncanon view which splits them by canon-ness (perhaps also an outdated classification) and the arc view, which groups by arc.

Related to #21.

[irisfofs]

I think the best way to improve this handling is to make the index-ness of a page not unique. Allow pages to opt in to getting all of the index information, and let them decide how to display stuff.

[irisfofs]

So instead of having a single main_index and arc_index, there can be more than one of each. Simple enough.

Furthermore, instead of splitting RPs up by canon/noncanon, we should dump them all into a single list. They could be split up again in Liquid if desired.
As for arcs... leaving them alone is fine for now. You can already not display the RPs that aren't arcs by editing the arc index page.