Issue: Variable interpolation is not working in array/list for placeholder #752

Describe the bug
When using a default placeholder that is an array, the variables are not interpolated inside.

To Reproduce
Create a new field in your contenttype with the following:

               {
                    "title": "aliases",
                    "name": "aliases",
                    "type": "list",
                    "default": [
                        "post/{{slug}}"
                    ],
                    "hidden": true
                }

You should get the following:

---
title: test-title
slug: test-title
aliases:
  - post/{{slug}}
---

Expected behavior
The slug variable should be rendered properly.

---
title: test-title
slug: test-title
aliases:
  - post/test-title
---

Note that using "default": "post/{{slug}}" properly renders "default": "post/test-title" so the problem is related to how array are managed.