Update the text for the about page, so translators can get started. Design is in flux.

props nacin, markjaquith, jorbin, jenmylo.
see #25603.

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Check if background core updates are supported on about.php.

see #25603, #22704.

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Remove redundant code. see #25603, #22704, [25833].

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In automatic background updates, standardize on 'update'.

New, final filter names:
 * auto_update_{$type} (plugin, theme, core, language)
 * automatic_updates_is_vcs_checkout
 * automatic_updates_disabled

New class name is WP_Automatic_Updater. Method names include update() and should_update().

see #22704.

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set_current_screen() is now unnecessary in Automatic_Upgrader_Skin, which is good because it breaks things in the network admin.

fixes #25622.

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