refactor: use MULTI_TENANT env variable, model concern instead of controller (#accepting_signups?)

app/views/sessions/new.html.erb

@@ -10,10 +10,10 @@
       </label>
     </div>
 
-    <% if single_tenant? %>
-      <p>Enter your email and we’ll get you signed in.</p>
-    <% else %>
+    <% if Account.accepting_signups? %>
       <p><strong>New here?</strong> <%= link_to "Sign up", new_signup_path %> to create an account. <strong>Already have an account?</strong> Enter your email and we’ll get you signed in.</p>
+    <% else %>
+      <p>Enter your email and we’ll get you signed in.</p>
     <% end %>
 
     <button type="submit" id="log_in" class="btn btn--link center txt-medium">