refactor(react-grid-material-ui): get rid of the Paper wrapper

[SergeyAlexeev]

BREAKING CHANGE:
 
To make Grid for Material UI more flexible we've stopped using the Paper component inside the Grid's layout.
 
It allows placing our Grid to an existing Paper with other components. For example:

<Paper>
  <Button>Products</Button>
  <Button>Customers</Button>
  <Button>Sales</Button>
  <Grid
    /* ... */
  >
    {/* ... */}
  </Grid>
</Paper>

[kvet]

Why do you write it here? There is DemoContainer for that purposes.

[SergeyAlexeev]

In this case, I wrap all elements in a demo with Paper. It makes sense for the Theming and Column Chooser ones. It looks like, I should wrap only Grid...

[kvet]

Please fix the following demos:

• featured-redux-integration (nested grid)
• featured-controlled-mode (grid in delete confirmation)