Fix vertical center (#4906)

app/assets/stylesheets/helpers.scss

app/javascript/components/rooms/CantCreateRoom.jsx

...ript/components/rooms/room/join/RequireAuthentication.jsx

app/javascript/components/rooms/room/join/RoomJoin.jsx

...script/components/rooms/room/join/RoomJoinPlaceholder.jsx

...ipt/components/users/account_activation/VerifyAccount.jsx

app/javascript/components/users/authentication/SignIn.jsx

app/javascript/components/users/authentication/Signup.jsx

...t/components/users/password_management/ForgetPassword.jsx

...pt/components/users/password_management/ResetPassword.jsx

...ipt/components/users/registration/PendingRegistration.jsx