Make changes to Rosetta for inactive translations

[samuelsidler]

As discussed on make/polyglots, we'd like to make some changes to the Rosetta theme that automatically kick in based on the current version number offered on each Rosetta site.

The following changes should be made when the version on a Rosetta site reaches current major version minus two (e.g., when 3.9.x is latest, 3.7.x or earlier):

• The download button should be removed.
• A banner should be placed across the top of the page with a yellow background that reads: "This translation of WordPress is currently inactive. If you’re interested in translating WordPress to <locale>, join the ​Polyglots team and find out how. ​Download the English (US) version instead."

All other links/information on the page should remain.

[nacin]

In 1059:

Rosetta: Add notice for inactive translations. see #487.

[nacin]

In 1060:

Rosetta: Don't use the native name on the inactive translation message. see #487.

[nacin]

This is also now applied to the language guesser on the wordpress.org homepage — it won't recommend a locale if it's two or more major versions behind. (Previously, it suggested locales that didn't even have a release yet.)

The same guesser is also on /download/ and /support/. The download page was already restricting its guesses to any locale that had the very latest release. The forums is based on which locales have forums.