* The maintenance apache site used bootstrap.min.css downloaded
  from the CDN. This is discouraged according to lintian:
  $ lintian-info -t privacy-breach-uses-embedded-file
* The canonical way to solve this issue is to use the corresponding
  debian package providing the referenced file, which does indeed
  exist: 'libjs-bootstrap'. In hawat, which this maintenance site
  replaces during downtime, corresponding css file is included
  statically, so adding another dependency just for maintenance
  site is an overkill. Both the static hawat version and the version
  in current debian's libjs-bootstrap are from Bootstrap v3.4.1,
  but the maintenance site is based on v4.3.1, so just download
  that and include it statically. The misversioning might be a typo
  and then either reusing the hawat's version or changing both
  to use the debian's packaged one might be a better way.

* The maintenance apache site used bootstrap.min.css downloaded
  from the CDN. This is discouraged according to lintian:
  $ lintian-info -t privacy-breach-uses-embedded-file
* The canonical way to solve this issue is to use the corresponding
  debian package providing the referenced file, which does indeed
  exist: 'libjs-bootstrap'. In hawat, which this maintenance site
  replaces during downtime, corresponding css file is included
  statically, so adding another dependency just for maintenance
  site is an overkill. Both the static hawat version and the version
  in current debian's libjs-bootstrap are from Bootstrap v3.4.1,
  but the maintenance site is based on v4.3.1, so just download
  that and include it statically. The misversioning might be a typo
  and then either reusing the hawat's version or changing both
  to use the debian's packaged one might be a better way.

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)