Use button type=submit instead of input.

This PR replaces all `input type="submit"` (generated by `submit_tag`) with `button type="submit"`.

Starting from IE7 they are the same, and button is more flexible as it can contain HTML. Let's use button everywhere then: http://stackoverflow.com/questions/7117639/input-type-submit-vs-button-tag-are-they-interchangeable

Note: Rails adds `type="submit"` by default to `button_tag`.