Assign H2 to Textbox Headers

[MichSpieler]

Feature Description

As a user, I would like to have textbox headers assigned a H2 heading by default.

Feature Use Case

From a network manager based on review by their Digital Accessibility Team: We are finding that a lot of our authors are using textboxes, which we love and believe is a great pedagogical and organizational tool, but most leave the text in the header box at paragraph level when many times it should be changed to H2.

Link to ticket (for internal reference)