FEATURE: Allow string theme settings to display with multiple lines

To use, add `textarea: true` to the theme settings.yml. For example:

```
my_setting:
  default: "some string"
  textarea: true
```

spec/fixtures/theme_settings/valid_settings.yaml

@@ -10,6 +10,10 @@ string_setting_02:
   min: 2
   max: 20
 
+string_setting_03:
+  default: "string value"
+  textarea: true
+
 integer_setting: 51
 
 integer_setting_02: