Create middleware for authentication checking

[Tomasz-Kluczkowski]

Add login required middleware in core.
Add list of exempt ulrs:

• '/'
• login
• signup
  Rest of urls require login.
  Make sure messages can be passed informing user what went wrong - look at: Add field for django.messages in base template #41

[Tomasz-Kluczkowski]

Middleware is in place now.
Tests needed + add the django messages usage to show to user they need to be logged in.

[Tomasz-Kluczkowski]

Need a list of urls per site. Run a loop and confirm for all but the three listed above the login is required.
See if all urls can somehow be derived from urls.py from all of the apps instead of having to manually add them.