Added edit subscriber hook

This is for compatibility with Scribite 5.
For now placed the hook in form/actions.tpl, and works when edit "Intro message" for the newsletter.

Note: Adding hook heeds upgrade routine, I placed it in upgrade to version 2.2.1 (not new one), as 2.2.1 is 1 hour old only. If somebody managed to download from Github in this short period, please to revert version in modules table to 2.2.0 bifore install to force upgrade.