Add mw-ui-button styles since old button styles have been scrapped

Also adding a helpful comment.

Leaving existing mw-mf-action-button class for now since this
provides the float left. Eventually we should migrate the styles
in MobileFrontend to use mw-ui-button as well and reduce the
redundant styles that are set. See also:
Change I59e8441f4c4e1d33bff33afcab6c6670d7b99634

Bug: 61490
Change-Id: I7e14b8eb522d8d7443fa917d97cb91773c6410f0