Add sidebar link to Ruby AU Slack channel

[sevenseacat]

When Len gets the permission bit sorted out, then we can make Slack public and more awesome!

[jomontanari]

Wow, for a small change that required a lot of files to be edited :/

:+1:

[sevenseacat]

Because all of the pages are generated statically, and the sidebar is included on every page, every single static page needed to be updated... I thought it was a mistake too, but apparently not!