lowercase username placeholder in Password Login and Registration Form

src/components/views/auth/PasswordLogin.tsx

@@ -357,6 +357,7 @@ export default class PasswordLogin extends React.PureComponent<IProps, IState> {
                     key="username_input"
                     type="text"
                     label={_t("Username")}
+                    placeholder={_t("Username").toLocaleLowerCase()}
                     value={this.props.username}
                     onChange={this.onUsernameChanged}
                     onFocus={this.onUsernameFocus}

src/components/views/auth/RegistrationForm.tsx

@@ -522,6 +522,7 @@ export default class RegistrationForm extends React.PureComponent<IProps, IState
             type="text"
             autoFocus={true}
             label={_t("Username")}
+            placeholder={_t("Username").toLocaleLowerCase()}
             value={this.state.username}
             onChange={this.onUsernameChange}
             onValidate={this.onUsernameValidate}