Changed page-notice__cta to just use btn and btn--secondary

[agliga]

Description

As per our discussion, changed the page-notice cta button to just use secondary button.
Changed styles for .btn and .fake-button to align with layout (in .page-notice)

Context

I had to force some layout changes, such as adding margin auto, or others in order to provide the different layouts for mweb and dweb.

Screenshots

[msendlakowski]

Per discussion on Slack. We should update the browser.json for notice so it includes the button dependency.

[ianmcburnie]

Looks good. Will leave it open until we see if Mike has any issues with this approach for success message.

[ianmcburnie]

@agliga Changes look good.

You're going to hate me, but I think I'm going to go with my original thought #452 (comment) and use a new block level class of .section-notice instead of page-notice--guidance. I just think even though right now the only difference is a border (which makes the css super simple), a section level notice is fundamentally different than a page level notice. It could deviate at any point to become more visually different than it is now (e.g. in the layout of its items), and if it does then a modifier could become tricky to work with. One other thing that sets it apart, is that it can host an overflow menu. This overflow menu might even necessitate that it becomes its own module at core-ui level. Don't worry though, I'll do the work this week in skin.

Anyhow, let's merge this one first.