LG-14642: Update design and copy of GPO 'letter on the way' email

[lmgeorge]

Why

• Align the 'letter requested' design and language with our current design requirements

How

• Rewrote the email template to utilize the new language and iconography.
• The template handles more complexity -- if the ServiceProvider is absent or has incomplete configuration details, we account for that and either hide or modify certain instructions and the call-to-action. This business logic is stored in a new presenter (Idv::ByMail::LetterRequestedPresenter).
• Replaced user_mailer.letter_reminder.* with new message properties under user_mailer.verify_by_mail_letter_requested.*
• Updated the Rails mailer previewer to include a dummy ServiceProvider and User with a pending profile to reflect the new content.

Additional Notes
As we utilize Rails.application.routes.url_helpers directly in the presenter, LocaleHelper#with_user_locale also needed to handle updating the locale within the url_options hash while also maintaining support for services that do not use url_helpers directly (e.g., SendProofingNotificationJob).

By default, it appears that Rspec mocks url_options with a frozen hash, so we simply override it for the purposes of testing. A scenario where url_options is defined but equal to nil is not supported. In the event this happens, we want the interpreter to throw an error because it likely indicates a serious misconfiguration or bug in the app.

changelog: User-Facing Improvements, IdV notifications, Redesign verify-by-mail 'letter requested' email notification

🎫 Ticket

Link to the relevant ticket:
LG-14642

[lmgeorge]

Author's note: currently waiting for program requirements to be met before this can be merged , but review and acceptance can still happen!

This work has been accepted, but still needs final business approval from program.