feat: invites (#967)

* stub invite endpoint

* feat: First pass invite UI

* feat: allow removing invite rows

* First pass: sending logic

* fix: label accessibility

* fix: add button submits
incorrect permissions
middleware flow error

* 💚

* Error handling, email filtering, tests

* Flow

* Add Invite to people page
Remove old Tip

* Add copy link to subdomain

app/components/VisuallyHidden.js

@@ -0,0 +1,13 @@
+// @flow
+import styled from 'styled-components';
+
+const VisuallyHidden = styled('span')`
+  position: absolute !important;
+  height: 1px;
+  width: 1px;
+  overflow: hidden;
+  clip: rect(1px 1px 1px 1px); /* IE6, IE7 */
+  clip: rect(1px, 1px, 1px, 1px);
+`;
+
+export default VisuallyHidden;