Update the meta descriptions & page titles thoroughly

[chriddyp]

For each chapter, make sure that the meta descriptions match the keyword concepts on the page. For example, if prevent_update is explained on that chapter, then include it in the meta description (e.g. "Advanced callbacks, including "prevent_update"")

Similarly update the titles to include these topics

[chriddyp]

Also add a test that makes sure that the meta descriptions are shorter than 160 characters.