This commit replaces the `full_name_required` setting with a new `full_name_requirement` setting to allow more flexibility with the name field in the signup form. The new setting has 2 options, "Required at signup" and "Optional at signup", which are equivalent to the true/false possibilities of the old setting, and a third option "Hidden at signup" that hides the name field from the signup form, making it effectively optional too.
New sites will have the "Hidden at signup" option as the default option, and existing site will continue to use the option that maps to their current configuration.
Internal topic: t/136746.
This reduces chances of errors where consumers of strings mutate inputs
and reduces memory usage of the app.
Test suite passes now, but there may be some stuff left, so we will run
a few sites on a branch prior to merging
