UX Improvement: Autofocus the topic field on edit

- After clicking on the pencil icon to edit a topic, the focus is not set on the input.
- This leads to cases where using keyboard shorcuts (e.g. cmd + left) leads to navigation instead.
- Searching around online, it seems the cleanest way to add a text-field that autofocuses is to create a component.
- I followed the [cookbook](http://emberjs.com/guides/cookbook/user_interface_and_interaction/focusing_a_textfield_after_its_been_inserted/) and did the most naive implementation.
- This focuses the text-field but at the start of the input. Setting selectionStart and selectionEnd solves this problem, but does not work on IE 8 and below.
Merge pull request #2878 from daemonsy/daemonsy_focus_topic_input

UX Improvement: Autofocus the topic field on edit