EU cookie law

[landswellsong]

I'd like the language selector to set a permanent cookie indicating one's language preference. However due to EU law we must have a clear consent of user to do so. Therefore I propose the following:

1. Start up page reads a cookie.
2. If no cookie is set, determines user's locale.
3. When the user selects something from the language combo box for the first time, display a banner with EU blablabla text and asking whether they want their language selection to be remembered.
4. If answer is yes, then set a persistent language cookie with language selected.
5. If answer is no, then set a session no_cookies cookie that prevents the dialog from popping up.

Note: "the first time" interpreted as: "user has neither language nor no_cookies set"

@lyssdod pls implement the banner. It will look ugly if I did it.

PS: applies to erint.science as well. I wager we want to make a separate option there to auto-redirect you to your prefferred language version.