Replace tag=hidden with cht:hidden=true in xforms

[alxndrsn]

We'll need to:

• support the new tag
• deprecate the old tag
• deprecate hidden_fields section of form properties files

[garethbowen]

Pushing back to 3.9.0 as there's no urgency on this one.

[abbyad]

This is a blocker for using the default ODK SMS format when sending reports as SMS because the attribute tag is used by both ODK's format, as well as our hidden fields.

The workaround for now is to use the custom SMS representation.

[garethbowen]

Moved to 3.12.0 to free up engineers to work on 3.11.0.

[abbyad]

Changing the backlog this is in since it primarily serves App Builders, who can more easily use the SMS features out of the box once we namespace the tag=hidden properly.