Add handbook name to each handbook

[samuelsidler]

In the mockup in #430, we have a handbook name across the top (in this instance "Polyglots Handbook"). We should add this ability to all handbooks.

[coffee2code]

In 800:

Handbook plugin: add support for defining and retrieving custom handbook name

• Add WPorg::get_name() to retrieve handbook name. Derives default if one isn't explicitly set.
• Add input for custom name via Settings -> General

See #439

[coffee2code]

In 802:

WP.org P2 theme: add handbook name to the top of handbook pages. Fixes #439