Template Validation big pieces

[Gregable]

Basic validation relaxations required for validating mustache templates:

• allow <template> tag
• allow corresponding <script> tag

Some lightweight validation errors not designed to catch all issues, but to guide the developer:

• disallow mustache characters { and } in data- attributes.
• disallow unescaped variables {{{var}}} and {{{&var}}} syntax in all attribute values
• disallow partials {{>partial}}} syntax in all attribute values.

Not a final implementation, but should be usable as written, I think.

Related to #892

[powdercloud]

LGTM