Make confirmation email for Welsh bi-lingual

[chao-xian]

What problem does this pull request solve?

When a user answers Welsh questions, the confirmation email now uses a bilingual Notify template. The mailer passes both English and Welsh personalisation variables (_cy variants for submission_date, what_happens_next_text, and support_contact_details) so each language section of the template receives the correct content. Template selection is driven by the explicit submission_locale parameter rather than the ambient I18n locale.

Trello card: https://trello.com/c/NLqWmwvf/2894-update-confirmation-emails-to-cater-for-new-welsh-form-submissions

Things to consider when reviewing

• Ensure that you consider the wider context.
• Does it work when run on your machine?
• Is it clear what the code is doing?
• Do the commit messages explain why the changes were made?
• Are there all the unit tests needed?
• Do the end to end tests need updating before these changes will pass?
• Has all relevant documentation been updated?

[stephencdaly]

Changes look good, we just need to make sure we're passing in the Welsh support details like we're passing in the Welsh "what happens next"

[stephencdaly]

Changes do what we need. I've suggested a few improvements to the tests for the additional logic added for the Welsh support details.

[github-actions]

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