Implement keyword input field component

[IreneStr]

Not in the image:

• The input filed should have the <label> Focus keyword
• The input field border should turn red when it contains comma-separated keywords. In that case, there should also be a warning message. See [Feature Request] Add warning that Focus Keyword is not comma separated wordpress-seo#9179 (comment) (scroll down for the latest version of the message).
• The input field should have an onChange prop so that we can change the state when there is a change.

Please branch from the feature/content-analysis-in-sidebar feature branch.

[afercia]

To make the message accessible, we should either use a speak message or do as in the MyYoast account section, where we use role="alert" on the element that appears on the page (see screenshot below):

The yellow box has a role="alert", which makes screen readers announce its content.