Pull search options from beacon

[gsfk]

Previously, the beacon tab in bento public has used the same discovery config settings as the Search tab to set metadata search options. Since Bento 18, beacon now knows its own search options, so these can be pulled from the beacon endpoint /filtering_terms instead.

• makes Beacon more self-contained and closer to its intended use:
  • it's now beacon's job to call katsu to figure out what the search options are, rather than leaving this to the web client
  • leaves open the possibility of filters that apply to other services (eg Takuan)
  • ideally the beacon UI should only need to communicate with beacon
• leaves open the possibility of working with non-Bento beacons

... and in general relying more on beacon /filtering_terms instead of direct calls to katsu will simplify the operation of the beacon network.

Also:

• made beacon config call conditional (it should have always been conditional)
• remapped some fields in beacon network filters (since they still follow "querySections" syntax)