Update of the http:// scheme

[michael-van-den-berg]

Replace the http:// scheme to https:// scheme. Except from the location where it is not needed or where no SSL Certificate is supported.

[fisharebest]

Some of these changes are good (thanks!) - but not all of them.

• some sites (e.g. http://www.phpgedview.net) do not appear to have a site at https
• the minified files *.min.css do not need to be changed. They will be updated automatically from the source *.css files whenever they are next rebuilt.
• as far as I can tell, the xlmns attribute in .xml files should not be changed. See Why does xmlns use http not https? w3c/svgwg#738
• vendor files (ckeditor) should not be changed as they will be overwritten when we upgrade. Perhaps create a PR for the upstream project?