Add sizes or em-sizing for custom form controls

[natcohen]

Just like input, dropdown, pagination and button; checkbox, toggle and slider should have a class (form-control-lg) to increase or reduce the size.

[mdo]

This will likely come in v5 with em-based sizing and consolidated form controls.

[natcohen]

@mdo When is v5 release planned?

[mdo]

It's our next focus after v4.3, so not too long from now I hope :).

[tmorehouse]

Duplicate of #27803

[mdo]

Duplicate of #27803, and also fixed in #28450.