Fix Facebook login issue #510

@Scott, @pupi1985

This fix is written and verified by http://question2answer.org/qa/user/Dan+D. in below thread http://question2answer.org/qa/56966/facebook-login-stopped-working-its-not-even-working-your-site?show=56992#a56992
also see https://github.com/q2a/question2answer/issues/510

I'm just submitting code changes.

Please check line no 593 for currentUrl. 

Thanks to Q2A user Dan D, and Github users valan, iamdand, cksagherian for this patch.

We now use qa-global.js for all the in-page JS (except for qa-admin.js
which regular visitors don’t need).

Clean up some of the JS code

Defaults to ‘anonymous’

Show/hide field and handle security on backend.

Fixes #470

Use onchange instead of onclick; add default permit in userfields

Avoid resetting $qa_content['script'] when already modified in admin_form()

Fixes #487

No need for this to be part of continuous integration.