Replace Twitter with Instagram (#803)

app/views/applications_mailer/confirmation.html.haml

@@ -7,7 +7,7 @@
     %p What happens now?
     %p
       We'd love to meet you. Please check our #{ link_to "Tumblr", "http://doubleunion.tumblr.com/" } and our
-      #{ link_to "Twitter", "https://twitter.com/doubleunionsf" } to find open events.
+      #{ link_to "Instagram", "https://www.instagram.com/doubleunionsf" } to find open events.
     %p
       Along with submitting an application, a qualification for membership is that you've met at least one member who is willing to say you seem like a reasonable potential member (such as that you'd respect the #{ link_to "base assumptions", BASE_ASSUMPTIONS_URL }, #{ link_to "anti-harassment policy", POLICIES_URL }, and responsibilities of being a member). Please come to events to meet some of us! Or if you'd prefer a quieter opportunity to meet just a couple members over a cup of tea, email #{ mail_to JOIN_EMAIL } and we'll set this up.
     %p