Replace javascript:; links with buttons.

Behavior improvements:

- middle click on the button does not open a `javascript:;` tab
- on hover browser does not show the link destination `javascript:;`

Appearance changes: none.

Also more semantic: links exist to redirect your browser somewhere else. This explains the better behavior.

Works because `button.btn type="button"` and `a.btn` are styled almost exactly the same in Bootstrap.

Just for reference, the elements that are affected are the "Reply" button and the "+" on diff comments:

![screenshot from 2014-09-19 15 24 22 gitlab javascript button to](https://cloud.githubusercontent.com/assets/1429315/4335883/3b7b88e4-4001-11e4-9e20-47984bad2aab.png)