Why this change?
This commit introduces an experimental `type: objects` theme setting
which will allow theme developers to store a collection of objects as
JSON in the database. Currently, the feature is still in development and
this commit is simply setting up the ground work for us to introduce the
feature in smaller pieces.
What does this change do?
1. Adds a `json_value` column as `jsonb` data type to the `theme_settings` table.
2. Adds a `experimental_objects_type_for_theme_settings` site setting to
determine whether `ThemeSetting` records of with the `objects` data
type can be created.
3. Updates `ThemeSettingsManager` to support read/write access from the
`ThemeSettings#json_value` column.
Similar to site settings, adds support for `refresh` option to theme settings.
```yaml
super_feature_enabled:
type: bool
default: false
refresh: true
```
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
