fix #2066 mixpanel wordcount property

fix #2066 mixpanel wordcount property
Actually on iOS, you can't tap the publish button if the post isn't publishable: https://github.com/wordpress-mobile/WordPress-iOS/blob/d008ffdc06b5cd8bd42aec366a38a4d0bdb2b805/WordPress/Classes/ViewRelated/Post/WPLegacyEditPostViewController.m#L509

I'll move the call to `trackSavePostAnalytics()`