When a theme setting of type `upload` has a default upload, it should return the URL of the specified default upload until a custom upload is used for the setting. However, currently this isn't the case and we get null instead of the default upload URL.
The reason for this is because the `super` method of `#value` already returns the default upload URL (if there's one), so we can't pass that to `cdn_url` which expects an upload ID:
c961dcc757/lib/theme_settings_manager.rb (L212)
This commit fixes the bug by skipping the call to `cdn_url` when we fallback to the default upload for the setting value.
Similar to site settings, adds support for `refresh` option to theme settings.
```yaml
super_feature_enabled:
type: bool
default: false
refresh: true
```
Previously theme setting descriptions were defined in the `settings.yml` file like this:
```
setting_name:
default: "My Default Value"
description:
en: "English description"
fr: "French description"
```
This commit allows developers to store the localised descriptions in the theme locale files instead:
```
en:
theme_metadata:
description: Theme Description
settings:
setting_name: "The localised description for setting_name"
```
This reduces chances of errors where consumers of strings mutate inputs
and reduces memory usage of the app.
Test suite passes now, but there may be some stuff left, so we will run
a few sites on a branch prior to merging
