Update and link to Pedalpalooza posters page

[carrythebanner]

While cleaning up images, I noticed that the Pedalpalooza Posters Past page isn't linked to from any active pages. Also, the most recent poster on the page is 2017.

We should clean this up:

• get high resolution images or PDFs for 2018 and onward
• use smaller images on the page, and link to the full-size, high-res file
• link to this page from the Pedalpalooza about page

[carrythebanner]

Also add a step in the Pedalpalooza playbook to add the current year's poster to the posters page.