Enable/disable the possibility of editing user's nickname or email

I added 2 settings to completely enable/disable the possibility of users to edit their nickname and/or email address in their profile page.

This is part of my modifications to allow for the integration of Discourse in a website using only the containing website users (basic user details and authentication, I let users manage their Discourse-only parameters like bio, avatars, mail preferences...). The session management and automatic creation of Discouse users is quite intrusive into the existing Discourse codebase (and not very "clean" yet), but the 2 settings I'm committing here are completely orthogonal to these big modifications. Since they might be useful for other workflows, I'm submitting this PR if you find it useful (of course I'm completely open to discussions about it!).