make display name mandatory (remove optional string)

_locales/en/messages.json

@@ -2318,8 +2318,8 @@
   "displayName": {
     "message": "Display Name"
   },
-  "enterOptionalDisplayName": {
-    "message": "Enter Optional Display Name"
+  "enterDisplayName": {
+    "message": "Enter Display Name"
   },
   "optionalPassword": {
     "message": "Optional Password"

ts/components/session/RegistrationTabs.tsx

@@ -309,7 +309,7 @@ export class RegistrationTabs extends React.Component<Props, State> {
           <SessionInput
             label={i18n('displayName')}
             type="text"
-            placeholder={i18n('enterOptionalDisplayName')}
+            placeholder={i18n('enterDisplayName')}
             value={this.state.displayName}
             onValueChanged={(val: string) => {
               this.onDisplayNameChanged(val);