Add CSS variables for cell height

[Storyyeller]

This is the same as #165 except that it also applies the variables to data-table-checkbox, which is required in order to make the table work when multiSelect is enabled.

The reason this is necessary is because normal CSS selectors such as the following don't work with the Shadow DOM, i.e. it breaks in any browser which implements web components without a polyfill.

iron-data-table data-table-cell {
  height: 100px;
}

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This change is 

[Saulis]

Hi!

Please see my comment here: #147 (comment)