SEO / Metadata implementation

[langphil]

User story:

1. As a producer I would like to enter relevant metadata into Publisher during the publishing process.
2. As a producer I would like this metadata to integrate nicely with the following: Facebook Open Graph, Twitter Cards, Google, anything else of relevance (discuss).
3. Implement Google Tag Manager < let's chat about this.

[gilesdring]

Design work being documented in https://github.com/theodi/shared/wiki/SEO-metadata

[gilesdring]

Fixed by

• Upversioning slimmer to 8.4.0 frontend-www#397
• Updates to default Open Graph markup frontend-www#398
• Adding article og:type frontend-www#399
• Updates to SEO tagging frontend-www#400
• Fixing og:image selector frontend-www#401

[gilesdring]

There are still some outstanding issues related to images. These are raised as separate issues.