feat: Advanced e-mail templating support

[landerss1]

This PR adds more advanced templating functionality to courier messages by

• Making Sprig functions available to templates
• Making the identity of the recipient available to templates
• Implementing support for nested templates in separate files

Related issue(s)

closes #834
closes #925

Checklist

• I have read the contributing guidelines.
• 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 green light (please contact
  security@ory.sh) 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

N/A

[zepatrik]

Looking pretty good already, thanks a lot 👍
I have some ideas how to improve it further 😉

[landerss1]

@zepatrik I've fixed everything in accordance with your suggestions in a separate commit, plus a few more changes I had to make to get your suggestions to compile and run.

Also, there are some issues with the e2e test, but I don't think that's something I caused:

Error: Timed out waiting for: http-get://127.0.0.1:4455/health, http-get://127.0.0.1:4456/health
    at MergeMapSubscriber.project (/home/circleci/project/node_modules/wait-on/lib/wait-on.js:132:31)
    at MergeMapSubscriber._tryNext (/home/circleci/project/node_modules/rxjs/internal/operators/mergeMap.js:67:27)
    at MergeMapSubscriber._next (/home/circleci/project/node_modules/rxjs/internal/operators/mergeMap.js:57:18)
    at MergeMapSubscriber.Subscriber.next (/home/circleci/project/node_modules/rxjs/internal/Subscriber.js:66:18)
    at AsyncAction.dispatch [as work] (/home/circleci/project/node_modules/rxjs/internal/observable/timer.js:31:16)
    at AsyncAction._execute (/home/circleci/project/node_modules/rxjs/internal/scheduler/AsyncAction.js:71:18)
    at AsyncAction.execute (/home/circleci/project/node_modules/rxjs/internal/scheduler/AsyncAction.js:59:26)
    at AsyncScheduler.flush (/home/circleci/project/node_modules/rxjs/internal/scheduler/AsyncScheduler.js:52:32)
    at listOnTimeout (internal/timers.js:557:17)
    at processTimers (internal/timers.js:500:7)

[aeneasr]

Looking great so far! I think we can extract the template interface with Execute into a common type, that would make code readability easier and also avoid duplication :)

[codecov]

Codecov Report

Merging #1859 (0612543) into master (305bb28) will decrease coverage by 0.05%.
The diff coverage is 67.70%.

@@            Coverage Diff             @@
##           master    #1859      +/-   ##
==========================================
- Coverage   75.19%   75.13%   -0.06%     
==========================================
  Files         292      292              
  Lines       15180    15241      +61     
==========================================
+ Hits        11414    11451      +37     
- Misses       2953     2965      +12     
- Partials      813      825      +12     

Impacted Files	Coverage Δ
selfservice/hook/verification.go	66.66% <0.00%> (ø)
selfservice/strategy/link/sender.go	71.27% <38.88%> (-5.56%)	⬇️
x/map_json.go	80.00% <55.55%> (-7.10%)	⬇️
courier/template/load_template.go	66.66% <71.69%> (+5.12%)	⬆️
courier/template/recovery_invalid.go	100.00% <100.00%> (ø)
courier/template/recovery_valid.go	100.00% <100.00%> (ø)
courier/template/stub.go	100.00% <100.00%> (ø)
courier/template/verification_invalid.go	100.00% <100.00%> (ø)
courier/template/verification_valid.go	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 305bb28...0612543. Read the comment docs.

[aeneasr]

Great job!