chore: make courier.TemplateType an enum

[jonas-jonas]

Related issue(s)

Checklist

• I have read the contributing guidelines.
• I have referenced an issue containing the design document if my change
  introduces a new feature.
• I am following the
  contributing code guidelines.
• I have read the security policy.
• 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 the approval (please contact
  [email protected]) from the maintainers to push
  the changes.
• I have added tests that prove my fix is effective or that my feature
  works.
• I have added or changed the documentation.

Further Comments

[codecov]

Codecov Report

Merging #2875 (20a85ea) into master (9eeedc0) will decrease coverage by 0.02%.
The diff coverage is n/a.

❗ Current head 20a85ea differs from pull request most recent head 453ab3f. Consider uploading reports for the commit 453ab3f to get more accurate results

@@            Coverage Diff             @@
##           master    #2875      +/-   ##
==========================================
- Coverage   75.92%   75.90%   -0.03%     
==========================================
  Files         303      303              
  Lines       18249    18249              
==========================================
- Hits        13856    13852       -4     
- Misses       3326     3329       +3     
- Partials     1067     1068       +1     

Impacted Files	Coverage Δ
courier/email_templates.go	68.42% <ø> (ø)
session/test/persistence.go	99.01% <0.00%> (-0.99%)	⬇️

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