favgroups: add stricter favgroup naming rules.

Don't allow favgroup names that:

* Start or end with underscores.
* Contain multiple underscores in a row.
* Contain asterisks or non-printable characters.
* Consist of only underscores.
* Consist of only digits (conflicts with `favgroup:1234` syntax).

Add a fix script that fixes favgroups that violate these rules and notifies the user.

test/unit/favorite_group_test.rb

@@ -58,4 +58,15 @@ class FavoriteGroupTest < ActiveSupport::TestCase
       assert_equal([], @fav_group.reload.post_ids)
     end
   end
+
+  context "when validating names" do
+    subject { build(:favorite_group) }
+
+    should_not allow_value("foo,bar").for(:name)
+    should_not allow_value("foo*bar").for(:name)
+    should_not allow_value("123").for(:name)
+    should_not allow_value("_").for(:name)
+    should_not allow_value("any").for(:name)
+    should_not allow_value("none").for(:name)
+  end
 end