[Grid] Use a unitless spacing API

[ifndefdeadmau5]

• I have followed (at least) the PR section of the contributing guide.

See the discussion about this PR here.

Closes #12925
Closes #13967
Closes #14280

Breaking change

In order to support arbitrary spacing values and to remove the need to mentally county by 8, we are changing the spacing API:

  /**
   * Defines the space between the type `item` component.
   * It can only be used on a type `container` component.
   */
-  spacing: PropTypes.oneOf([0, 8, 16, 24, 32, 40]),
+  spacing: PropTypes.oneOf([0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]),

Going forward, you can use the theme to implement a custom Grid spacing transformation function: https://material-ui.com/system/spacing/#transformation.

Deprecation

theme.spacing.unit usage is deprecated, you can use the new API:

    [theme.breakpoints.up('sm')]: {
-     paddingTop: theme.spacing.unit * 12,
+     paddingTop: theme.spacing(12),
    },

Tip: you can provide more than 1 argument: theme.spacing(1, 2) // = '8px 16px'

[eps1lon]

Are the visual differences intended? https://www.argos-ci.com/mui-org/material-ui/builds/40261

updated