FIX: make theme import's public key field readonly (#6424)

`disabled` attribute prevents the user from clicking or selecting in the
control whereas `readonly` does not.

app/assets/javascripts/admin/templates/modal/admin-import-theme.hbs

@@ -26,7 +26,7 @@
           {{#if publicKey}}
           <div class='public-key'>
             {{i18n 'admin.customize.theme.public_key'}}
-            {{textarea disabled=true value=publicKey}}
+            {{textarea readonly=true value=publicKey}}
           </div>
           {{/if}}
           {{/if}}