Hannah Vivian Shaw

While perusing your docs, I spotted a broken linked to MadeWithML. Nothin' too surprising, just a missing `.com`. This PR fixes that!

Move Send Webmentions to ActiveJob

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This PR does the next chunk of #1548, and the first which is an async enqueued job vs. a timed cron job. There were some larger changes in this one....

When I build the book locally, I find that Figure 2.1 comes out looking pretty odd:  I also get this error message in the terminal while building: ``` LaTeX...

When building locally, [in section 2.4 (pg. 42)](https://github.com/Introduction-to-Autonomous-Robots/Introduction-to-Autonomous-Robots/blob/d7c43fa75c5821193684324a20143de3a04c704c/chapters/kinematics-coordinatesystems.tex#L2-L4), I find that the QR code which links to [this video on coordinate systems](https://www.youtube.com/watch?v=klBJi-MEeNQ) is misplaced. It covers up the text like...

[#88] Add `Cache-Control` headers for Apple touch icons

1

This PR addresses issue lobsters/lobsters#1614 by updating the Nginx config with cache headers for Apple touch icons.