Use absolute image url for open graph image meta tag

[donni106]

Description

The og:image meta tag should have an absolute url to an image as value.
Currently Golden Cobra renders a relative url programmatically.

Steps to reproduce

The current rendering result for the og:image meta tag is:
<meta property="og:image" content="/system/images/ID/FORMAT/FILE">

Affected code lines if helpful

In this lines the first image of an article is used, if no manual url is given:
https://github.com/ikuseiGmbH/Goldencobra/blob/17fdf33f5e94277fed998acc185ff7e4d4871f26/app/models/goldencobra/article_concerns/meta_tag.rb#L39-L44

Expected results

The rendering result for the og:image meta tag should be:
<meta property="og:image" content="http(s)://WEBSITE/system/images/ID/FORMAT/FILE">