docs: don't render ERB

[shawnbot]

This marks all of our remaining fenced code blocks that contain ERB interpolations (<%= ... %>) as erb rather than html so that we don't render broken HTML. I replaced all instances of current_user.to_s with probot, but there are a ton of octicon and avatar helpers that still need to be "migrated" over to HTML (with React? /shrug).

I've also replaced a bunch of random image URLs with https://github.com/{username}.png avatar URLs.

TODO

• Update the base branch to release-10.10.4 once docs: link tests #645 lands