Make subject configurable using go template

[lsjostro]

Related issue

#128

Proposed changes

Checklist

• I have read the contributing guidelines
• I confirm that this pull request does not address a security vulnerability. If this pull request addresses a security
  vulnerability, I confirm that I got green light (please contact hi@ory.sh) from the maintainers to push the changes.
• I signed the Developer's Certificate of Origin
  by signing my commit(s). You can amend your signature to the most recent commit by using git commit --amend -s. If you
  amend the commit, you might need to force push using git push --force HEAD:<branch>. Please be very careful when using
  force push.
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation within the code base (if appropriate)
• I have documented my changes in the developer guide (if appropriate)

Further comments

[aeneasr]

This looks very good and can be merged as is. Could you please update the keto_warden section to reflect this change/enhancement?

[aeneasr]

Thanks!

[lsjostro]

Thank you! 😄