Add fr and de hero images (Fixes #713)

[MelissaAutumn]

Fixes #713

Pinging @jobisoft and @babolivier for feedback as I do not speak German, and my highschool french only gets me so far. (No rush on the feedback!)

[babolivier]

French looks good to me, translations seem to match what's in Pontoon. A couple of nitpicks on the label names:

• the translation for "Todo" is a bit awkward (it would translate to "do" rather than "to do"), "À faire" would be more fitting
• the translation for "Upgrade" is correct, but I think "Mise à jour" would feel a tiny bit more natural

[jobisoft]

Referring to the text on: https://github.com/thunderbird/thunderbird-website/blob/777264f59720df0aa4e7dc0cb3cec71b951ca6df/media/img/l10n/de/thunderbird/base/home/banner-high-res.png

The german translation "Lass uns fliegen! Es ist Zeit für Thunderbird 128." for "Let's fly! It's time for Thunderbird 128." is a literal translation, word by word. But the german translation does not seem to have the intended meaning. What are you trying to express?

• Let's go! It is time for Thunderbird 128. -> "Lass uns loslegen! Es ist Zeit für Thunderbird 128."

@kaie : What do you think?