Language Editor: Add key creation teaser

[resamsel]

In the language editor with no keys (initial state), no content is being shown. A teaser message should be inserted, with the option to create a key on-the-fly. After creating, the user should stay on the language editor page, but with the new key selected on the sidebar.

Definition of Done

• Teaser message with key creation button is shown
• After creation, the key should popup on the list on the left, and it is preselected
• A UI-test exists that verifies the DoD