Track when translations need to be updated

[annaritz]

Is your improvement request related to a problem? Please describe.
As we add more tutorials and networks, and people can revise existing tutorials, we should think about how to track and manage these changes.

Describe the solution you'd like

Some ideas:

• When a tutorial is updated, trigger an email to all authors (including translation authors) notifying them of this change. (Could cause email bloat, especially if people keep submitting typo fixes). Maybe a "notify authors" button?
• Have a yellow warning icon that displays on translations that have an earlier posting date than the current tutorial. "Warning: contents may be out of date."
• Trigger a GitHub issue when a tutorial is updated and tag the other authors (requires all authors to have github accounts and access to the Tutorials repo).