Fixed .png

Changed .PNG for .png in some images
Merge pull request #1878 from al3mon/docs_tutorial_images

Fixed .png
Add missing messages block to base.html
Support activating accounts by admin users.

This patch supports activating users via the admin console. The workflow goes like this:

 * Administrator sets REGISTRATION_OPEN, ACCOUNT_APPROVE_REQUIRED to True and configures an email gateway.

 * Users try to register the site and are told the account is pending approval.

 * All users with the is_staff flag are emailed with a link to the Profile edit page.

 * As soon as any of the staff approves the user, he or she receives an email saying the account is active and a link to log in the site.

This depends on geonode-user-accounts 1.0.9 and has defaults in settings.py, local_settings.py and documentation production.txt about the new option.