Add Slack landing page

[ZimbiX]

If I send someone a link to join the Ruby AU Slack (either https://ruby.org.au/slack or https://ruby.org.au/my/slack_invite), when they open the link, all they see is a login page. I think it would be better if we changed all the links to https://ruby.org.au/slack, and have that show a page saying something like:

Ruby Oceania Slack

Our Slack org is open only to registered members of Ruby Australia. To register, simply create an account here on ruby.org.au, then click the link below.

(Big button with Slack logo and text saying 'Join', greyed out when not logged in, linking to https://ruby.org.au/my/slack_invite)

That would also permit a more descriptive link preview than this:

[leesheppard]

Task

• Create a static page for https://ruby.org.au/slack that will have a slack logo with hyperlink to Slack invite page for RubyAU
• Link will only be available to logged in members, greyed out otherwise with an invitation to Join as a member displaying
• Remove any references of https://ruby.org.au/my/slack_invite