Add author's avatar to the individual issues

[BasThomas]

As per https://twitter.com/daveverwer/status/1139299506472796161 😃

We use avatars on the authors page already, so that should be a good start: https://github.com/SwiftWeekly/swiftweekly.github.io/blob/464c1e6994f3075890b7b95687ec458d12eed942/authors.html#L19

[jessesquires]

Awesome idea.

We should make this a new includes/ component to avoid duplicate code.