schnarch: simple editor added

This commit adds basic editing of the source data. Editing the data file will
generate a diff on the server such as to preserve previous versions of the data
file.

Signed-off-by: Thilo Fromm <github@thilo-fromm.de>
editor: make diff ignore line endings

Signed-off-by: Thilo Fromm <github@thilo-fromm.de>