Expand context when the value is an URI and expasion was requested in the config

[elrayle]

Adds:

• expands context for URIs to optionally include label and id

Fixes:

• group label improperly passed default to I18n
• clean up some comments
• extracts parts of longer methods into private methods to avoid rubocop complaints and/or to keep DRY

Examples:

Regular context (from previous PR)

{
    property: "Authoritative Label",
    values: [
        "Science films"
    ],
    selectable: true,
    drillable: false
},

Expanding URI values in context (with this PR)

{
    group: "Hierarchy",
    property: "Narrower",
    values: [
        {
            uri: "http://id.loc.gov/authorities/genreForms/gf2011026416",
            id: "gf2011026416",
            label: "Nature films"
        }
    ],
    selectable: true,
    drillable: true
},