Convert TFA plugin forms to use subforms

Problem/Motivation

Currently we run a somewhat complicated/restricted setup where we:

• Do not use subforms which limits how how we can pass data between form handlers
• We have our form validators and form submit handlers return bool responses
• Expose additional API to obtain data that could be handled by the plugin.

By converting to subforms we can allow the plugins to directly handle form related errors and move closer to a standard FormInterface workflow that

Steps to reproduce

N/A

Proposed resolution

Switch to subforms

Remaining tasks

User interface changes

API changes

Yes, validatation methods and submit methods will no longer return bool, and possible removal of further unneeded API.

Data model changes

None