Add config options to customize where uploads are stored, and how image URLs are generated.
* Add `media_asset_file_path` option to customize where uploads are stored.
* Add `media_asset_file_url` option to customize how image URLs are generated.
* Remove the `enable_seo_post_urls` config option. The `media_asset_file_url` option
should be used instead to include the tags in the image URL.