Improve registration

* Add ability to set the user display name
 * Use display name to guess the username/channel name
 * Add explanations about what is the purpose of a username/channel name
 * Add a loader at the "done" step

shared/extra-utils/users/users.ts

@@ -73,7 +73,7 @@ function registerUser (url: string, username: string, password: string, specialS
 
 function registerUserWithChannel (options: {
   url: string,
-  user: { username: string, password: string },
+  user: { username: string, password: string, displayName?: string },
   channel: { name: string, displayName: string }
 }) {
   const path = '/api/v1/users/register'
@@ -84,6 +84,10 @@ function registerUserWithChannel (options: {
     channel: options.channel
   }
 
+  if (options.user.displayName) {
+    Object.assign(body, { displayName: options.user.displayName })
+  }
+
   return makePostBodyRequest({
     url: options.url,
     path,