Prep shared API URL util for use on external sites

[ErisDS]

refs #5942, #6150

There were a few key problems I was looking to solve with this:

• Introduce a single point of truth for what the URL for accessing the API should be
• Provide a simple way to configure the utility (much like a true SDK)

As of this commit, this utility is still automatically available in a Ghost theme.
To use it on an external site, the code would look like:

<script type="text/javascript" src="http://my-ghost-blog.com/shared/ghost-url.min.js"></script>
<script type="text/javascript">
ghost.init({
   clientId: "<your-client-id>",
   clientSecret: "<your-client-secret>"
});
</script>

To achieve this, there have been a number of changes:

• A new apiUrl function has been added to config, which calculates the correct URL. This needs to be unified with the other url generation functions as a separate piece of work.
• The serveSharedFile middleware has been updated, so that it can serve files from / or /shared and to substitute {{api-url}} as it does {{blog-url}}.
• ghost-url.js and ghost-url.min.js have been updated to be served via the serveSharedFile middleware
• ghost-url.js has been changed slightly, to take the url from an inline variable which is substituted the first time it is served
• {{ghost_head}} has been updated, removing the api url handling which is now in config/url.js and removing the configuration of the utility in favour of calling init() after the script is required
• {{ghost_head}} has also had the meta tags for client id and secret removed
• tests have been updated