Rename the setting "SIGNATURES_STORAGE_BACKEND" to
"DOCUMENT_SIGNATURES_STORAGE_BACKEND" and the setting
"SIGNATURES_STORAGE_BACKEND_ARGUMENTS" to
"DOCUMENT_SIGNATURES_STORAGE_BACKEND_ARGUMENTS". This is
to differentiate these from the settings of the django_gpg
app.
Signed-off-by: Roberto Rosario <Roberto.Rosario@mayan-edms.com>