Stop moving session language ahead of reqLangs

[pbrisbin]

Yesod.Core.Handler.languages checks first for a language set in the user's
session, prepending that value to YesodRequest{reqLangs}, so it is respected
above all else if present.

For context, reqLangs itself also includes the session, but just later in
line:

langs' = catMaybes [ lookup langKey gets -- Query _LANG
                   , lookup langKey cookies     -- Cookie _LANG
                   , lookupText langKey session -- Session _LANG
                   ] ++ langs                    -- Accept-Language(s)

In #1720, it was raised that allowing the session (something implicitly present
for any request) to override a query parameter (something explicitly given on
that request) is surprising.

We decided (without knowing what order reqLangs was doing) that query, cookie,
session, accept was best and languages should be changed to do that.
Conveniently, this just makes languages equivalent to reqLangs, so that is
what this patch does.

---

I'll do this in a fast-follow:

• Bumped the version number
• Documented new APIs with Haddock markup

After submitting your PR:

• Update the Changelog.md file with a link to your PR
• Check that CI passes (or if it fails, for reasons unrelated to your change, like CI timeouts)
• Send a message to the mailing list about the order change

[snoyberg]

Can you also add a comment in the Haddocks that the behavior of the function changed, and in which version?

[pbrisbin]

Before I fire off the yesodweb message, are you good with 1.6.18.9 or do you want to do 1.6.19.0?

[snoyberg]

Let's do 1.6.19.0

[pbrisbin]

This is all set, just waiting on the Approve :)

[snoyberg]

Thanks!

[snoyberg]

And released to Hackage