Create collections of pinned plots

[epompeii]

With pinned plots implemented (#232), it is now possible to have a single dashboard-like interface of pinned plots.

The UI for this is currently just a single large page where each plot has a configurable index/rank.

However, it would probably be useful to have groups of pinned plots. A pinned plot could belong to and be ranked within a collection. This would allow for multiple different names boards in the dashboard-like interface.

This may end up being a better solution for users than tagging individual dimensions (#240) in order to easily track and compare results.