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