don't use deprecated theme spacing

interface/src/forms/ManageUsersForm.js

@@ -26,18 +26,18 @@ import { withAuthenticationContext } from '../authentication/Context';
 
 const styles = theme => ({
   loadingSettings: {
-    margin: theme.spacing.unit,
+    margin: theme.spacing(0.5),
   },
   loadingSettingsDetails: {
-    margin: theme.spacing.unit * 4,
+    margin: theme.spacing(4),
     textAlign: "center"
   },
   button: {
-    marginRight: theme.spacing.unit * 2,
-    marginTop: theme.spacing.unit * 2,
+    marginRight: theme.spacing(2),
+    marginTop: theme.spacing(2),
   },
   table: {
-    '& td, & th': { padding: theme.spacing.unit }
+    '& td, & th': { padding: theme.spacing(0.5) }
   },
   actions: {
     whiteSpace: "nowrap"