UI: Add notices for published, updated, save failed

[jasmussen]

Splitting this out from #707. The flow for publishing and saving is soon in a good place.

We have, however, a need for a few edgecase notices:

• Published successfully
• Updated successfully
• Scheduled successfully

... as well as failure states:

• Publishing failed
• Updating failed
• Scheduling failed

... and also for draft saving and auto saving:

• Save failed

Save Failed

Let's make this part of the "Save" area on the left:

Publish/Updating/Scheduling notices

We'll probably want notices after all (see also #486), because thankfully we were able to test for a while a design where the notice itself was integrated in the button. Not only did this feel ephemeral, but it made the button itself feel more complex. It also doesn't work that well if we ever get to exploring a dropdown button as suggested in #708.

So here's how notices could look:

[afercia]

Worth noting that all these notices would need an equivalent audible feedback for screen reader users. As a general rule: any "visual" notice needs to be communicated also non-visually.

[melchoyce]

FWIW, we have a separate set of notice styles in core for notices on a white background:

If the inspector/sidebar is open, would the notice overlap or appear to the left of it?

[jasmussen]

Let's totally use the colored versions as they are.

[melchoyce]

Cool — they all use the .notice-alt class:

<div class="update-message notice inline notice-warning notice-alt">
    <p>Notice Message.</p>
</div>