Sharing: improve the fallbacks for "via" on twitter sharing button

Before, the button always outputted "@jetpack"
I believe this was caused by the extra `if ( ! empty( $account ) )` check before to look for a `publicize_twitter_user` custom field.

This commit introduces 2 fallback levels:
- Was the post created by an author with a connected Publicize account?
- If not, did the site admin enter a jetpack-twitter-cards-site-tag value?
- If not, fallback to "@jetpack"