Don't act busy on the login page for moving your cursor

If you were in the username field and simply tabbed out without entering anything, the form would become "busy" and not let you submit. We should only be doing this if we have work to do, like .well-known discovery of the homeserver.

src/components/structures/auth/Login.js

@@ -236,12 +236,13 @@ module.exports = React.createClass({
     },
 
     onUsernameBlur: async function(username) {
+        const doWellknownLookup = username[0] === "@";
         this.setState({
             username: username,
-            busy: true, // unset later by the result of onServerConfigChange
+            busy: doWellknownLookup, // unset later by the result of onServerConfigChange
             errorText: null,
         });
-        if (username[0] === "@") {
+        if (doWellknownLookup) {
             const serverName = username.split(':').slice(1).join(':');
             try {
                 const result = await AutoDiscoveryUtils.validateServerName(serverName);