No indication of invalid session

[DJankauskas]

If the site session is rendered invalid (or does not exist), the user is allowed to remain without any indication until a privileged mutation fails. Validation calls need to be added, with a redirect to the sign-in page on invalid query.

Steps:

• add validateSession functionality in the session redux code
• add customized text and redirect capability to the SignInPage component
• redirect to sign-in on invalid session detected

[AndySok]

For devs: almost done, check if it's actually completely done or not