Add new application members to an empty application.

The link button to add a new application member to an app that did not
have any ("empty state")  was not wired up. This:

- Moves the two steps of the new member modal into macros so that they
  can be reused with different button/link types
- Wires the "invite a new team member" button on the application team
  page
- Makes a minor CSS tweak so that icons nested under `.empty-state` in
  the modal aren't enormous

styles/components/_empty_state.scss

@@ -10,7 +10,7 @@
     padding: 8rem ($gap * 4) 0;
   }
 
-  .icon {
+  > .icon {
     @include icon-size(50);
     @include icon-color($color-gray-light);
   }