Added Snap WRITABLE_PATH.

Thanks to xet7 !

snap-src/bin/mongodb-control

@@ -26,12 +26,10 @@ fi
 
 # Not in use. If uploads directory does not exist, create it.
 # Wekan will store attachments there.
-if [ ! -d "$SNAP_COMMON/wekan-uploads" ]; then
-   mkdir "$SNAP_COMMON/wekan-uploads"
+if [ ! -d "$WRITABLE_PATH" ]; then
+   mkdir "WRITABLE_PATH"
 fi
 
-export WRITABLE_PATH=$SNAP_COMMON/wekan-uploads
-
 # Alternative: When starting MongoDB, and using logfile, truncate log to last 1000 lines of text.
 # 1) If file exists:
 #if test -f "$SNAP_COMMON/mongodb.log"; then