Add '@' to the list of characters removed from radio/checkbox ID.

This character can easily show up in email addresses and can cause
invalid HTML.

Refs #2733
BC fix for checkbox ids

Revert to previous behavior for id generation of checkboxes, This commit
can be reverted for 2.5+ is desired

Fixes #2733
Merge pull request #2766 from AD7six/2.4.x-bc-form-fix

BC fix for checkbox ids