Change: Default type of OcButton

We've changed the default type of buttons rendered by `OcButton` to `button`.
Browsers otherwise assume they are of type `submit` which leads to very unexpected
behavior in forms, especially as we use `OcButton` in a lot of (not so obvious) places
for a11y reasons.


https://github.com/owncloud/owncloud-design-system/pull/2009
