Framework: remove <Flag> component

[mtias]

This removes the <Flag> component in favour of <Notice isCompact>.

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Test

This should cover all existing use cases of Flag. cc @scruffian @breezyskies

This is the one I could spot myself:

May need some tweaks for the line.

[breezyskies]

Overall, this looks good. Noticed this one seems to be missing the icon (should be lock):

[mtias]

The TLDs look like this:

[breezyskies]

👍 Privacy protection icon looks good now.

[mtias]

Awesome, thanks for looking.

[gziolo]

@breezyskies: onClick doesn't work for privacy protection None icon, should we bring back this behavior or remove callback:

<Notice
    isCompact
    status="is-warning"
    icon="notice"
    onClick={ this.goToPrivacyProtection }>
    { this.translate( 'None', {
        context: 'An icon label when Privacy Protection is disabled.'
    } ) }
</Notice>

@mtias: would it be ok to add onClick handler to Notice component or should we wrap it with another element?

[mtias]

I think it should be wrapped, since Notice is neither a button nor a link in itself.

[breezyskies]

I think it should be wrapped, since Notice is neither a button nor a link in itself.

Opened a PR for this in #1740.