Changed the text on discussion tab based on replies/comments

- Separated *discussion* code from ```node_ajax_view.html```.
- Created ```discussion.html``` as separated template which contains discussion related ```HTML``` and ```JS``` code.

**Now the text on tab will be:**
- If node object has comments/replies on it then the text **Join Discussion (<No. of comments>)** will be shown.
- If node object does not have comments/replies on it then the text **Begin Discussion** will be shown.
- Also tool-tip on tab's text is provided for more details.