When the page is loaded over HTTPS but Piwik thinks it is loaded over HTTP, explain in a notification how to solve issue

[mattab]

from #6703 (comment)

• Display a notification to user:

  • when in JavaScript in Piwik reporting UI, we can detect whether https is current protocol used,
  • and when Piwik server side thinks that http is used,
  • The notification explains to user something such as:

  You are currently viewing Piwik over secure SSL connection (using https), but Piwik could only detect that a non secure connection was used. To make sure Piwik correctly requests your content over HTTPS, you may edit your config.ini.php file and configure your proxy settings, or you may add below [General] the line: assume_secure_protocol=1. Learn more

[mattab]

Note: assume_secure_protocol=1 is currently auto-detected during installation only and will be set in config when:

• JavaScript detected https
• and Piwik server did not detect https

Problem lies in the fact that user may change the Piwik setup after installation (ie. change the proxy setup, or SSL proxy), and this may break Piwik. Showing this notification will help users restore Piwik working securely over SSL.