Add volunteer tree recognition training

[umonkey]

Adds a new UI at /learn to train oneself in recognizing trees.

[sentry-io]

Suspect Issues

This pull request was deployed and Sentry observed the following issues:

• ‼️ TypeError: $.get(...) is null /learn View Issue
• ‼️ TypeError: $.get(...) is null /learn View Issue
• ‼️ TypeError: (intermediate value).learnTitle is not a function /learn View Issue

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