Make seat count editable on checkout page

[vdavid]

Context:
This is what the Checkout page looks like:

The user can change the number of seats with the +/- buttons.

Problem:
The user has no way to just change the number in the text field using their keyboard. They have to use the +/- buttons, which is cumbersome.

Suggested solution:
Replace the <div> with an input field.

[chenkc805]

Updating this to P1, this is very annoying to do and hinders seat growth