Add meta tags for client_id & client_secret

[ErisDS]

This PR adds the meta tags which will contain client_id and client_secret. Three conditions must be met for this to appear:

1. there must be a client with the slug ghost-frontend
2. the client must have status === 'enabled'
3. the public API labs flag must be checked

Here's what it looks like:

There are 2 additional meta tags:

<meta property="ghost:client_id" content="ghost-frontend">
<meta property="ghost:client_secret" content="c6deeb75cfe5">

With this, using the public API via ajax no longer requires you to go fishing in your database. Assuming you have <ul id="result"></ul> somewhere on your page, the following will fetch a list of your tags:

<script type="text/javascript" src="https://code.jquery.com/jquery-1.11.3.min.js"></script>
<script type="text/javascript">
var apiUrl = 'http://localhost:2368/blog/ghost/api/v0.1/',
        clientId = $('[property="ghost:client_id"]').attr('content'),
        clientSecret = $('[property="ghost:client_secret"]').attr('content'),
        url = apiUrl + 'tags/?limit=all&client_id=' + clientId + '&client_secret=' + clientSecret;

function onSuccess(data) {
    var $result = $('#result');
    $.each(data.tags, function (i, tag) {
        $result.append('<li><a href="/tag/' + tag.slug + '/">' + tag.name + '</a>&nbsp;</li>');
    });
}
$(document).ready(function () {
    $.ajax({url: url, type: 'get'}).done(onSuccess);
});

---

refs #5942

• refactor ghost_head to use Promise.props (settle is going away and this is easier)
• add a new call to fetch the frontend client, if it exists
• add meta tags for the client_id and client_secret on all pages
• don't include the meta tags if the client is not enabled, or if the labs flag is not set