Subscription form: do not display the logged in user's email

Can be overwritten with the jetpack_auto_fill_logged_in_user filter
Fixes #694
Subscriptions: Fix spacing

See https://github.com/jeherve/jetpack/commit/686ed49ceda02d4f49d66f1028a4523e9f882437#commitcomment-6888035
Subscriptions: missing whitespace

See https://github.com/Automattic/jetpack/pull/725/files#r20790518
Merge pull request #725 from jeherve/subs_noemail

Subscription form: do not display the logged in user's email address by default