AO3-7406 Move gift exchange notifications to GiftExchangeMailer

[Smuzzy-waiii]

Pull Request Checklist

• Have you read "How to write the perfect pull request"?
• Have you read the contributing guidelines?
• Have you added tests for any changed functionality?
• Have you added the Jira issue number
  as the first thing in your pull request title (e.g. AO3-1234 Fix thing)
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Issue

https://otwarchive.atlassian.net/browse/AO3-7406

Purpose

This PR moves the following notifications related to gift exchange from UserMailer to GiftExchangeMailer:

• assignments_sent_notification
• assignment_default_notification
• invalid_signup_notification
• potential_match_generation_notification

Credit

Smaran Jawalkar (he/him)

[Smuzzy-waiii]

@slavalamp I don't have Jira perms as a first time contributor, could you please set the ticket to In-Progress? 😄

[slavalamp]

hi, thanks for making this pr (and for fixing the url comments for my mailer previews)!

i've set the jira issue to "in review" since that's what an "in progress" status gets automatically changed to when you submit a pr. if it's not ready to be reviewed yet, you should be able to mark this pr as a draft

i've only briefly looked through your changes for now, so just letting you know that you can use i18n-tasks to see why the i18n test failed, and you still have leftover invalid_signup_notification and potential_match_generation_notification methods in the user mailer. let me know if you need help with anything else here

also you can check that "have you added tests" checkmark - you're not adding anything new here, but you are updating the tests so they can test the changes you're making

[Smuzzy-waiii]

Hi @slavalamp! Thank you so much for your quick help and guidance

I have removed the leftover invalid_signup_notification and potential_match_generation_notification methods in the user mailer. Turns out that is what was causing the i18n tests to fail too

Please do let me know if there is anything else needed from my side, including any steps I need to take to get access to Jira (if that is needed at this stage)