feat: add MailType as email template parameter

[ryanhalliday]

What kind of change does this PR introduce?

Tiny feature, include the MailType in mail template data so that you can build URLs more dynamically. Rely less on strings.

What is the current behavior?

Users have to type out magic strings and have to know type manually for each mail type

What is the new behavior?

{{ .MailType }} can be used in email templates

Additional context

Introduces const in template.go - I know these strings are used elsewhere (verify.go) but they couldn't be cleanly imported and deal with different concerns. Maybe unifying them is an idea one day.

I had to write a different docs PR here so this sort of fixes that: supabase/supabase#21466

TODO?

Likely needs docs update for this section of the Supabase dashboard:

Let me know if you want that PR too, should be easy.

[ryanhalliday]

lmk if I need to fix the conventional commit thing or if that is fine to do at merge.

[hf]

Looks pretty good!

[hf]

Can you add some examples to the docs? Or at least here in comments / PR description, so that we can add it in official Supabase docs?

[ryanhalliday]

TODO

• Update docs
  • https://github.com/supabase/supabase/blob/master/apps/docs/content/guides/cli/customizing-email-templates.mdx
  • https://github.com/supabase/supabase/blob/master/apps/docs/content/guides/auth/auth-email-templates.mdx
• Update UI https://github.com/supabase/supabase/blob/master/apps/studio/components/interfaces/Auth/AuthTemplatesValidation.tsx

Will try add these some time this week 👍

[ryanhalliday]

Docs PR added, let me know if any of it needs tweaks!

[ryanhalliday]

@hf resolved merge conflicts in this and docs PR