Improve UX of Wiki Editor page

- Add a welcome message for first time contributor
- Add Help icons with links to relevant sections in Editor documentation
- Add explanations in page for Tags, Revision Comment and Attachments
- As Tags are now important for MDN search engine, moves them above Revision Comment
- Add Editor Save/Cancel buttons at the bottom of the form

Checked with Ali, Holly, Janet, and more… :smiley: 

**Screenshot:**
![new-editor](https://f.cloud.github.com/assets/188748/2365947/9a49b728-a6e7-11e3-9953-38e4cfa60efa.png)