Adjust "Leave a reference" email template

[nabramow]

Before merging this, we need to merge the frontend part in order to have a link to add #5508 .

If a host accepted a request, then the surfer did not stay for whatever reason, we want to allow this as an option in the emails we send, reminding them to leave a reference.

Acceptance Criteria:

• Adjust the two templates for this: app/backend/templates/v2/reference_reminder.txt and app/backend/templates/v2/reference_reminder.mjml
• the email will just have two buttons, one to go write a ref and the "yes" will be pre-ticked in the reference flow. and one that says "didn't stay with {firstName}?" and that will pre-tick the "no". (Doesn't have to be the exact language do what fits).