Support the opengraph image meta tag

[mmattel]

Referencing:
#396 (Revise head-meta-opengraph.hbs)
owncloud/docs#4639 (Add attribute necessary for embedding opengraph images in docs-ui automatically)

This PR adds opengrap images automagically dependent on the docs product.
Before merging this PR, the refrerenced docs PR should get merged to make the necessary attribute available.

• Improving the contains.js script to cover pure contain matches and word-contain matches
  The latter is used for og:image. Note that the js code adoption for handelbars drove me nuts and has been tested intensively
• Fixing the intend of opengrap and twitter so that the output is aligned. Used for better readaility
• Adding handlebar code to the template to add og:image automatically based on the component name including a fallback if no match has been found

Examples:

Server

    <meta property="og:image" content="https://doc.owncloud.com/opengraph-server.png" />
    <meta property="og:image:alt" content="The ownCloud Server documentation" />

Desktop

    <meta property="og:image" content="https://doc.owncloud.com/opengraph-desktop.png" />
    <meta property="og:image:alt" content="The Desktop Client documentation" />

Branding

    <meta property="og:image" content="https://doc.owncloud.com/opengraph-default.png" />
    <meta property="og:image:alt" content="The Branded Clients documentation" />

Note that Branding does not have an own entry in the attributes list, therefore the default image is used. The Branding content will be integrated soon into the particular client documentation.

[xoxys]

I would suggest prettifying the HTML in the public folder after the Antora build instead. Anyway, approved.

[xoxys]

The downside of this variant is that every formatter on earth will revert it or the files need to be excluded from auto-formatting.

[mmattel]

prettifying the HTML in the public folder after the Antora build

Agreed, but lets do this in another step.
If we found a usable one and have implemented it, it is easy to remove this intends