[Theme] Tailor Social media sharing options

Behaviour:

• Clicking on a social media sharing option should open a new tab -- moved to indigo-totem#14 (closed)
• Add Twitter, Facebook and Instagram handles
  • Twitter
  • Facebook
  • Instagram -- can be done in new issue if necessary
• Add mailing address
  • Write which mailing address to use in this issue
  • Add it to the configuration
• Tailor the text (can it be different per language?)