"Find nearest metadata" functionality

[waxlamp]

i.e., a function that walks up the Girder hierarchy, looking for the nearest ancestor that includes a given metadatum.

We may want a more specific version of this functionality that also includes something like a "merge" operation: for the annotation tracker tasklist, there might be a general task list at the folder level, with customizations at the item level. However, this feature has not been specifically needed yet, so we can feel free to skip it for now, or file an issue around it for later.