Validation for CiviMail UI

[x] - Name
[x] - From
[x] - Recipients
[x] - Subject
[x] - Schedule
[x] Mutually exclusive fields: Reply-To vs Track Replies
[x] Require one or both: HTML, Plain text
[x] Enforce required tokens
[x] Wizard: Each step should be a subform which must be validated before advancing to the next step