Improve data loading in Browser

[graft]

The Browser component must load data from Magma and Timur in order to render; it must post manifests to Archimedes and retrieve consignments. Currently this loading happens through a rather baroque set of functions on the Browser and various brute-force methods for gathering data (e.g. pulling all plots and manifests from Timur).

Instead the Browser should make use of a Loading module that ascertains what data is required, what data is missing, and makes the appropriate requests to various data sources, updating the store as it proceeds until the Browser renders correctly.