Implemented simple maintenance information page mechanism.

Sometimes the downtime due to the upgrade or error can be noticable to the users, so the well behaving application should have some kind of maintenance mode to inform the users. This commit contains simple web page with capabilites of variable substitutions (to insert separately generated timestamps about the duration of the downtime), an example site configuration for Apache web server and updated documentation page about sustem upgrade. (Redmine issue: #4277)