CHECKOUT-1189 : Adding support to Order Confirmation page in Stencil

[snaderiBC]

What?

1. Adding Order Confirmation page to Themes so that along with Checkout page it can also be customized.
2. Adding the feature customized_checkout to activate the feature for themes.
   I've excluded above to prevent any racing conditions when merging related PRs.
   After below PRs are merged, I'll create another PR with just the theme feature to prevent any unwanted issue in production.

Related PRs

https://github.com/bigcommerce-labs/ng-checkout/pull/394
https://github.com/bigcommerce/bigcommerce/pull/17826

Screenshots (if appropriate)

[snaderiBC]

Ping: @lord2800 @mcampa @icatalina @jordanwink201 for review

[mcampa]

Please add a line to CHANGELOG.md file describing your change

[snaderiBC]

Also, @mcampa I've named and renamed everywhere for the final page of check out to be Order Confirmation.
Looks like there have been already a page called order-complete.html. So, I suggest to remove that and replace it with what I've added to this PR so that it is consistent everywhere.
Also, is dash a convention? order-confirmation.html instead of orderconfirmation.html ?

[mcampa]

LGTM

[icatalina]

LGTM 👍