As baggy theme was removed and material is the only remaining theme, we don't need a theme switched anymore.
So:
- move all `*.twig` files from the material theme folder to the root
- remove useless translations
Since password are linked to username it was hard to change username while checking that the password is the good one.
Instead of doing crazy things to achieve that, I act that user won't be able to update username.
Also, username (and email) must be unique, since people logged in using username and can request a new password using email.
This should fix#1149
