Provide editorconfig to avoid incosistent indention

[Rotzbua]

The html files have mixed indention. Some use tabs some use spaces.

This can be fixed by using the config from:
https://editorconfig.org/

It is supported by most editors and by github.

The commit formatting the files can be added to the .git-blame-ignore-revs to keep git-blame history clean.

[t-paul]

I tried with prettier and just .editorconfig which seems to look ok after a number of HTML fixes. I'll probably start with just the top level '*.html'.