Move unique OAuth username creation to client-side

Now, on OAuth signup form, we create a unique username with random appended string only if there's a conflict.
Previously, this was always happening during the Slack OAuth flow. This has the benefit of preventing username collisions for all OAuth providers.

oauth_slack.go

@@ -13,8 +13,6 @@ package writefreely
 import (
 	"context"
 	"errors"
-	"fmt"
-	"github.com/writeas/nerds/store"
 	"github.com/writeas/slug"
 	"net/http"
 	"net/url"
@@ -167,7 +165,7 @@ func (c slackOauthClient) inspectOauthAccessToken(ctx context.Context, accessTok
 func (resp slackUserIdentityResponse) InspectResponse() *InspectResponse {
 	return &InspectResponse{
 		UserID:      resp.User.ID,
-		Username:    fmt.Sprintf("%s-%s", slug.Make(resp.User.Name), store.GenerateRandomString("0123456789bcdfghjklmnpqrstvwxyz", 5)),
+		Username:    slug.Make(resp.User.Name),
 		DisplayName: resp.User.Name,
 		Email:       resp.User.Email,
 	}

pages/signup-oauth.tmpl

@@ -130,7 +130,7 @@ var $aliasSite = document.getElementById('alias-site');
 var aliasOK = true;
 var typingTimer;
 var doneTypingInterval = 750;
-var doneTyping = function() {
+var doneTyping = function(genID) {
     // Check on username
     var alias = $alias.el.value;
     if (alias != "") {
@@ -153,6 +153,11 @@ var doneTyping = function() {
                     $aliasSite.className = $aliasSite.className.replace(/(?:^|\s)error(?!\S)/g, '');
                     $aliasSite.innerHTML = '{{ if .Federation }}@<strong>' + data.data + '</strong>@{{.FriendlyHost}}{{ else }}{{.FriendlyHost}}/<strong>' + data.data + '</strong>/{{ end }}';
                 } else {
+                    if (genID === true) {
+                        $alias.el.value = alias + "-" + randStr(4);
+                        doneTyping();
+                        return;
+                    }
                     aliasOK = false;
                     $alias.setClass('error');
                     $aliasSite.className = 'error';
@@ -170,6 +175,14 @@ $alias.on('keyup input', function() {
     clearTimeout(typingTimer);
     typingTimer = setTimeout(doneTyping, doneTypingInterval);
 });
-doneTyping();
+function randStr(len) {
+    var res = '';
+    var chars = '23456789bcdfghjklmnpqrstvwxyz';
+    for (var i=0; i<len; i++) {
+        res += chars.charAt(Math.floor(Math.random() * chars.length));
+    }
+    return res;
+}
+doneTyping(true);
 </script>
 {{end}}