Attempt to remove duplication in `_button.html.haml` and `_new_button.html.haml`

The following discussion from gitlab-ce!30961 should be addressed:

• @pslaughter started a discussion:

  preference: Wow! _button.html.haml and _new_button.html.haml are almost the exact same. 🤔 Can we create a follow up issue to resolve this duplication? Maybe a community contributor will pick it up 😄

There is a lot of duplication between _button.html.haml and _new_button.html.haml

They are basically buttons of different sizes, but maybe the code can be merged and DRYed up.

_button.html.haml	_new_button.html.haml
Used in user profile notifications	Used in the header of a project or group page