Fix traditional Instagram embeds

[PaulAdamDavis]

Now that Ghost doesn't create Instagram embeds itself, users will instead use the embed code Instagram provides, into a HTML card.

This PR adds some highly specific styles that add vertical margins and horizontally centre the iframe that the <script> that the provided embed injects. The injected iframe has inline styles that necessitate the proposed change.

<iframe
    class="instagram-media instagram-media-rendered"
    id="instagram-embed-2"
    style="background: white; max-width: 540px; width: calc(100% - 2px); border-radius: 3px;
        border: 1px solid rgb(219, 219, 219); box-shadow: none; display: block; margin: 0px 0px 12px;
        min-width: 326px; padding: 0px;" class="instagram-media instagram-media-rendered"
    id="instagram-embed-2"
    src="..."
    allowtransparency="true" 
    allowfullscreen="true"
    frameborder="0"
    height="745"
    data-instgrm-payload-id="instagram-media-payload-2"
    scrolling="no">
</iframe>

Before

After