Current aggregator selected as changed by fallthrough logic is not reflected in the options page

[machawk1]

Largely driven by the changes in #316 via #367, Mink has an option of a series of aggregators for archival sources. If the first one is down, the logic will change to querying the second one.

I am not sure whether the runtime logic persists for subsequent runs but the fall-through, subsequent aggregator is not indicated as the default in the options tab.

This requires some logical work but also UI considerations, e.g.,

• Should aggregators be sorted or re-arrangeable?
• Should there be some logic for a random aggregator of those that Mink knows are alive and accessible?

[machawk1]

The aggregator options are encoded in a <select> in options.html. This aligns with the hard-coded options in content.js, both of which ought to be relocated to some sort of external config file to be referenced in both cases.