Improve confirm subscription and subscription confirmed emails

[simison]

• [ ] Ensure preview text looks good in Gmail
• [ ] Move big manage/unsubscribe CTAs to footer similar to new posts template
• [ ] Remove "whoa slow down" links to notifications; we already link to subscription management and this is an transactional email anyway.
• [ ] Update subject to "You're now subscribed to {site name}!"

[github-actions[bot]]

This issue has been marked as stale. This happened because:

• It has been inactive for the past 6 months.
• It hasn’t been labeled `[Pri] BLOCKER`, `[Pri] High`, `[Type] Feature Request`, `[Type] Enhancement`, `[Type] Janitorial`, `Good For Community`, `[Type] Good First Bug`, etc.

No further action is needed. But it's worth checking if this ticket has clear reproduction steps and it is still reproducible. Feel free to close this issue if you think it's not valid anymore — if you do, please add a brief explanation.