secure-cookies: Allow to use secure HTTPS cookies (Django setting)

Cf https://docs.djangoproject.com/en/1.4/ref/settings/#std:setting-SESSION_COOKIE_SECURE
Merge pull request #160 from YoshidaKS/EDX-344

EDX-344: secure-cookies: Allow to use secure HTTPS cookies (Django setting)
EDX-640: get session expiry from settings
EDX-645: hide grade-distributions from instructor dashboard
EDX-612: fix UI of inline discussion for iphone
Merge pull request #161 from YoshidaKS/EDX-640

EDX-640: get session expiry from settings
Merge pull request #162 from hkawasaki/EDX-645

EDX-645: hide grade-distributions from instructor dashboard
Merge pull request #163 from hkawasaki/EDX-612

EDX-612: fix UI of inline discussion for iphone
EDX-634: Fix max size of upload file for submission