Updated Stripe JS to load async in head
Updated Stripe JS to load async in head

no issue

- Stripe JS is added to a theme via ghost_head if a Stripe account is connected to members enabled site
- Previously, the script was not loading async which blocked the main thread, changes the script load to async to avoid rendering block
- Members script is already being loaded with `defer` so does not block the main thread

bitjson/Ghost,ErisDS/Ghost,e10/Ghost,ErisDS/Ghost,janvt/Ghost,janvt/Ghost,TryGhost/Ghost,TryGhost/Ghost,e10/Ghost,TryGhost/Ghost,ErisDS/Ghost,bitjson/Ghost