Convert Javascript links to buttons.

Same rationale as: https://github.com/gitlabhq/gitlabhq/pull/7863

Found with `git grep '#'`.

Refactored for every place except projects.

Only touched buttons that look like buttons, not links. Those should work with `.btn.btn-link` but for some reason didn't.

There were several dropdown ones factored with:

    find . -iname '*.haml' | xargs perl -lapi -e "s/%a.dropdown-toggle.btn{href: '#', \"data-toggle\" => \"dropdown\"}/%button.dropdown-toggle.btn{type: 'button', 'data-toggle' => 'dropdown'}/"

We should really DRY up those dropboxes before they start diverging.