Design sign-up email

[john-gom]

Add modified HTML templates from https://github.com/openfoodfacts/openfoodfacts-auth/tree/c833f3d370ca528069c7d4d335ea92d0698602d9/theme/base/email to https://github.com/openfoodfacts/openfoodfacts-auth/tree/c833f3d370ca528069c7d4d335ea92d0698602d9/theme/off and modify messages to match OFF wording.

Note: Keycloak doesn't provide a "sign-up" message per-se, but we want to use mail verification for new accounts, which could be used as a surrogate "welcome" mail

[john-gom]

I've noticed that the email is always in English and does not reflect my user's preferred language. @hangy is that the same for you?

[hangy]

Just tested on https://world.off.hangy.de/ and the German locale I selected for my account seems to work:

[john-gom]

Ok. I wonder if the message strings are coming from a different place than for the themes. I was using a dummy language XX that isn't in the standard keycloak list of locales