md-settings: Tweak padding for action/cancel buttons and subpage header.

Add left right padding to action/cancel buttons.
Reduce bottom padding for settings subpage header.
Also removed a todo for button opacity(saw a new value in specs).

Link to specs in the bug.

TEST=manual
BUG=703998
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2831963004
Cr-Commit-Position: refs/heads/master@{#466545}
2 files changed