UI config to toggle whether user email shows up in Explore Users (#336)

UI config to toggle whether user email shows up in Explore Users (#336)

* UI config to toggle whether user email shows up in Explore Users

* Recommendation made by @tboerger
https://github.com/go-gitea/gitea/pull/336/commits/66a1c59fe730eff019ce100673c6800cce7d102d#r94122732

* fixed typo, rename ShowUserEmailInExplore to ShowUserEmail

* Fixed typo merged conflict

* Hide email in the user profile page

if you are active ShowUserEmail
ref https://github.com/go-gitea/gitea/pull/336#issuecomment-269843725

* Please replace MustBool() with MustBool(true)