Foreach xml translation logic

[elohanlon]

For rendering an array of $projects info in field exports, we need to add a new foreach translation rule.

// Once we call foreach on the $projects variable, each iteration runs in the context of a single project's json, which looks like this:

{
  "display_label" => "Great Project",
  "uri" : "http://www.example.com/project/123"
}

// The translation logic could look like this:
{
  'foreach' : {
    "field" : "$projects",
    "xml_translation" : [
      {
        "element": "mods:relatedItem",
        "attrs": {
          "type": "host",
          "displayLabel": "Project"
        },
        "content": [
          {
            "element": "mods:titleInfo",
            "content": [
              {
                "element": "mods:title",
                "content": "{{display_label}}"
              }
            ]
          },
          {
            "element": "mods:location",
            "content": [
              {
                "element": "mods:url",
                "content": "{{uri}}"
              }
            ]
          }
        ]
      }
    ]
  }
}

To make this code more generic, it could also apply to normal dynamic fields. This would allow us to do render nested fields like name_role:

// Imagine we're in the context of a name element
{
  'foreach' : {
    "field" : "name_role",
    "xml_translation" : [
      {
        "element": "mods:role",
        "content": [
            {
                "element": "mods:roleTerm",
                "attrs": {
                    "type": "text",
                    "valueURI": "{{name_role_term.uri}}",
                    "authority": "{{name_role_term.authority}}"
                },
                "content": "{{name_role_term.value}}"
            }
        ]
      }
    ]
  }
}