Initial auto update implementation

Fixes #1260

- Added update button to ugly debug UI.
- Button checks for updates by parsing the location header on the
ghost-latest.zip redirect.
- If an update is available, a modal confirmation window is presented to
confirm the installation of the update.
- Installation process includes creating the `content/tmp/` folder
(unless it already exists), downloading the zip to that folder,
extracting the zip on top of the current installation and then deleting
the zip from `tmp`.
- Note the update process still requires you to manually run `npm
install` and `npm update` and restart the server. See the comments in
the associated item for more information to that end.