MM-12885: Add UI widgets AdminHeader and FormattedAdminHeader.

[grundleborg]

Summary

Add UI widgets AdminHeader and FormattedAdminHeader.

This adds 2 new widgets:

• AdminHeader, which is a widget that makes the Admin Console page header which can take any components or values as children.
• FormattedAdminHeader, which is a child widget that can be used where the AdminHeader component is to contain a single localised (markdown) string.

These are not the most complicated components in existence, but they are
distinctly UI widgets that can be separated from the code, and were
needed for the code I was writing at the present.

This also adds a dependency on sinon for testing.

Ticket Link

https://mattermost.atlassian.net/browse/MM-12885

Checklist

• Ran make check-style to check for style errors (required for all pull requests)
• Ran make test to ensure unit and component tests passed
• Added or updated unit tests (required for all new features)

[grundleborg]

@jasonblais added you to review as this PR introduces a new dependency: sinon.

[jasonblais]

Thanks George - switching PM review to @wiersgallak as she owns legal and licensing now :)

[wiersgallak]

@grundleborg, as a common practice it looks like we add these dependencies to the NOTICE.txt file. Can you please do that for this one as well? Thanks!

[grundleborg]

Made @saturninoabril suggested change, so this PR no longer introduces a new dependency, so removing @wiersgallak from review request.

[jespino]

This "breaks" the translation extraction mechanism. There are solutions. The simplest one would be add the t function call in the id attribute, something like:

<FormattedAdminHeader
  id={t('analytics.system.title')}
  defaultMessage='System Statistics'
/>

Other solution is to add the new FormattedAdminHeader to the extraction tool, but if you add a lot of this, you will need to add each one to the translations extraction tool.

Other option is try to make the tool smarter, something like try to extract from any component with name starting by "Formatted" with attributes id and defaultMessage, and if is needed, add specific cases for specific components.

[grundleborg]

I think for now I'm going to go for the second option: adding FormattedAdminHeader to the i18n extraction script. If more components like this get added, we can consider the third option then. I don't think the first option is acceptable.