Add AssignEditor component

packages/component-dashboard/src/components/AssignEditor.js

+import React from 'react'
+import { Menu } from 'xpub-ui'
+import { withJournal } from 'pubsweet-component-xpub-app/src/components'
+
+const editorOption = editor => ({
+  value: editor.user,
+  label: editor.name
+})
+
+// TODO: select multiple editors
+
+const AssignEditor = ({ journal, project, team, teamType, addUserToTeam }) => (
+  <Menu
+    value={team ? team.members[0] : null}
+    label={journal.roles[teamType]}
+    options={journal.editors[teamType].map(editorOption)}
+    placeholder="Assign an editor…"
+    onChange={user => {
+      addUserToTeam({
+        team,
+        teamType,
+        name: journal.roles[teamType],
+        group: 'editor',
+        project,
+        user
+      })
+    }}/>
+)
+
+export default withJournal(AssignEditor)