Warn users of impending license expiration

[cjcenizal]

I chatted with @jasontedor and we identified some ways to improve the UX when a license is about to expire.

Global warning banner

When the license is 7 days out from the expiration date, we can render a global warning banner similar to the existing "expired license" banner. Suggested content:

Your license will expire in <NUMBER_OF_DAYS> days
Contact your administrator or update your license for continued use of features

License Management Space-awareness

We should look into making License Management Space-aware, so that administrators can hide it from users who shouldn't have to worry about licenses. This will help prevent accidental clicks on the "Revert to Basic" button.

License Management privileges check

Jason's looking into the required privileges for the license API. We can institute a hasPrivileges check and render a "Missing privileges" warning in place of the UI when necessary.

[elasticmachine]

Pinging @elastic/es-ui (Team:Elasticsearch UI)

[jasontedor]

Relates elastic/elasticsearch#60562.