Define UTF-8 as default charset

[Robbert]

What places do we need to define this?

• [x] .editorconfig:
• [x] .htaccess
• [x] Check all files for valid UTF-8 encoding

[*]
charset = utf-8

AddCharset UTF-8 .js .json .html htm .css .svg .xml .xsl .xsd .rng .md

Anywhere else?

[Robbert]

Encoding checking now works, because eclint checks the encoding defined in .editorconfig.