users: don't allow users to choose reserved names.

Don't allow users to choose names that conflict with search syntax, like `any` or `none`, or names
that impersonate user levels, like `Admin`, `Moderator`, `Anonymous`, etc.

test/unit/post_vote_test.rb

@@ -102,7 +102,7 @@ class PostVoteTest < ActiveSupport::TestCase
 
     context "deleting a vote by another user" do
       should "leave a mod action" do
-        admin = create(:admin_user, name: "admin")
+        admin = create(:admin_user)
         vote = create(:post_vote, post: @post, score: 1)
 
         vote.soft_delete!(updater: admin)
@@ -114,7 +114,7 @@ class PostVoteTest < ActiveSupport::TestCase
 
     context "undeleting a vote by another user" do
       setup do
-        @admin = create(:admin_user, name: "admin")
+        @admin = create(:admin_user)
         @vote = create(:post_vote, post: @post, score: 1)
 
         @vote.soft_delete!(updater: @admin)

test/unit/user_test.rb

@@ -205,6 +205,13 @@ class UserTest < ActiveSupport::TestCase
         user.save
         assert_equal(["Name is not allowed"], user.errors.full_messages)
       end
+
+      should_not allow_value("any").for(:name)
+      should_not allow_value("none").for(:name)
+      should_not allow_value("new").for(:name)
+      should_not allow_value("admin").for(:name)
+      should_not allow_value("mod").for(:name)
+      should_not allow_value("moderator").for(:name)
     end
 
     context "searching for users by name" do