gravatar settings and global env

- Added the `enable_gravatar` to store the gravatar images
- Seed the system user with default custom image(hack)
- Rename `automatically_download_gravatars` to `automatically_refresh_gravatars`, also change the scheduled job to refresh gravatars
- Using GlobalSetting.gravatar_url(DISCOURSE_GRAVATAR_URL) to download gravatars.
- Avatar selector UI now reflects the site settings.
- FIX: the uploaded custom avatar can not be save

Many space to refactor avatars. I think I can refactor avatars in 1.3. For now, that's it