Add SITE_URL

[stuartmaxwell]

Related to #93, would be useful to have access to the SITE_URL. I could use the Sites framework to help with this, but that might overcomplicate things right now. I'm thinking to have a SITE_URL variable that would be set in the DJPRESS_SETTINGS object.

[stuartmaxwell]

Note that both of my publishing plugins needed access to a site URL which had to be configured in the plugin settings.