Draft: Remove updateField in favour of v-model

• Remove updateField() from WishlistIntake.vue, because after we switched to v-model the check of this.wish[ field ] !== value was never working because this.wish had already been updated to value at that point. That method was setting this.formChanged = true, and this change gets rid of that as well as this.formSubmitted, with the function of those two being taken up by this.allowCloseWindow (i.e. setting it to null to start with, and then giving it a value when the form is changed, signifying that navigating away should be blocked).
• Remove "FIXME: this code may not be needed anymore?" from updateTitle, because that code is needed.
• Remove "@todo Handle this nicer from handleSubmit, because that's now tracked in T371950.