Add separate OtpDeliveryMethodForm for IdV

[jmhooper]

Why: In the IdV flow, we know the number will be a US number so we
don't have to validate delivery methods against the phone number.
Additionally, we don't need to update the user's delivery preference.
For these reasons it makes more sense for the IdV flow to have it's own
form.

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[jmhooper]

Pictured here: Me begging for merge conflicts against #2430

[monfresh]

Here's a wild idea: what if we got rid of this otp_delivery_selection_form altogether? The only thing it's doing is making sure the method is either "sms" or "voice", and the only way it would not be is if someone deliberately manipulated the HTML on the page before submitting the form. How about if we default to "sms" if something other than "sms" or "voice" is submitted?

[monfresh]

Or maybe default to not do anything and display the delivery selection page again.

[jmhooper]

It makes some changes to the idv session in the other PR. The delivery preference gets saved there instead of added as a url param. We could move that logic to the controller, but idk if we want to?

[jmhooper]

Actually, I'm looking at my new code now and I do actually already set it in the controller. Hmm, yeah we can try removing it completely.

[jmhooper]

Though another downside of not have a form is that we don't have a result object to feed to analytics here.

[jmhooper]

@monfresh: Anything to add here?

[monfresh]

All good. I think we decided to leave this here when we discussed this offline.

[monfresh]

love the sax

[monfresh]

LGTM