Use tab setting from .editorconfig when viewing files

Many projects use EditorConfig to keep the settings between the various contributors the same. Over time, it has become a well supported and much used standard. All major editors have support for it.

When you view a file or a diff in GitLab, tabs are always shown as 8 chars, even if the .editorconfig file says otherwise. This can make viewing a file of diff bit difficult.

When viewing, AFAICS indent_size and tab_width are the only really relevant properties in the .editorconfig file.

This issue is related to #13352 (moved), but that is about adding support to the editor/web ide. This one is about the viewer.