Fix Medium's post thumbnail

[tico88612]

Which problem is this PR solving?

• Resolves #677

Description of the changes

• Fix Medium's post thumbnail
  • Solution following: https://medium.com/@kartikyathakur/getting-those-thumbnails-from-medium-rss-feed-183f74aefa8c

How was this change tested?

• Manual test

Checklist

• [x] I have read https://github.com/jaegertracing/jaeger/blob/master/CONTRIBUTING_GUIDELINES.md
• [x] I have signed all commits
• [ ] I have added unit tests for the new functionality
• [ ] I have run lint and test steps successfully
  • for jaeger: make lint test
  • for jaeger-ui: yarn lint and yarn test

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