luci-app-filemanager: flexible width, dark mode, refinements

luci-app-filemanager: flexible width, dark mode, refinements

Automatic width and height for the file-list,
automatic width for text editor, hex editor
and help window. Justification is that user
can adjust width using browser window and
allows responsive width to function.
Added support for dark mode, previously
was hard-coded for light mode and caused
a visual conflict with using bootstrap which
adapts to both modes. Minor fixes to buttons,
heading, nav bar, borders. Small style tidying.
Further tested across all luci-themes. Added
fallback CSS if a theme does not have dark
mode, and enclosed most open CSS classes
such as .selected, as it was affecting other
elements outside of the file manager.

Closes #8537.

Signed-off-by: Joshua Criss <redacted>