Drop SnackbarContent and ContentProps

[iamhosseindhv]

• className prop is now used to customise styles applied to the content of snackbar (e.g. backgroundColor).
• If you were using className to apply styles to Snackbar component, you should now do so using classes.root.

<SnackbarProvider
-  className={classes.snackbar}
+  classes={{
+    root: classes.snackbar
+  }}
>

• ContentProps prop is not supported anymore. Here are alternative ways to pass the same data to snackbar component. For any other
  scenario, you should pass you own custom content.

<SnackbarProvider
-  ContentProps={{
-    action: () => <button>dismiss</button>,
-    'aria-describedby': 'some-value',
-  }}
+  action={() => <button>dismiss</button>}
+  ariaAttributes={{
+    'aria-describedby': 'some-value'
+  }}
>

• If you have customised MuiSnackbarContent through Material-UI theme object, these changes won't automatically
  reflect within notistack. You can however, use className prop to apply your customisations.

const theme = createMuiTheme({
  overrides: {
    // no effect within notistack
    MuiSnackbarContent: {
      root: {
        fontSize: '1rem',
      },
    },
  },
});