update_status 5.x-2.0-rc

This is a (hopefully final) release candidate (RC) before the official 5.x-2.0 release. At this point, no new features will be added. Changes since 5.x-2.0-beta2:

• #124715: Added setting for email notifications when updates are available.
• #154868: Fixed bugs and inconsistencies with security update warnings.
• #153757: Renamed the "Warning threshold" setting to "Error threshold".
• #149079: A few CSS tweaks and fixes for recommended version:
  • Renamed the CSS class to "version-recommended-strong" to indicate it's the strongly recommended version among many (the old name, "version-recommended-not-latest" isn't good since "Latest" isn't the only possible thing we're distinguishing against).
  • Fixed classes so that if we're recommending a security release, it gets the subtle "zebra" stripe, too.
• #154689: Restored the CSS classes on the per-project settings table, (and made them consistent across all rows), but made them more specific to avoid name conflicts.
• Fixed minor glitch in the logic for when to display the 'Development version'.
• Fixed bug in hook_version_alter() invocation for CVS deploy integration.
• Updated screenshot to show off recent UI and functionality improvements.