Add theme_settings to stores
As we are going to implement theming support in BitcartCC store, see bitcartcc/bitcart-store#319, we need to extend the API to allow storing css url.
CSS url for the theme is just an url which will be loaded by the frontend. 
Ideally in the future we will allow to host css right on the server and use a permanent link to server storage, for that #161 and file storage support should be implemented, but that's for another issue
To add `theme_settings`, we need a new JSON field for Store model.
https://github.com/bitcartcc/bitcart/blob/712d7ffb829cfee517ffa95c056ab37310870a85/api/models.py#L266-L270
it should be marked as json field for our validators, see how `checkout_settings` field is implemented and referenced for guide on how to add it
A new scheme should be added, which will only have `store_theme_url` and `admin_theme_url` variables (type str)

After that, admin panel should be updated to allow editing new field.
https://github.com/bitcartcc/bitcart-admin/blob/05350eac9daa926649aba3ab4d200b8ebdc0fa22/pages/stores.vue#L31-L41
Add it in a similar way to `checkout_settings`, with a new icon. 