Improve Template Previews

[oudeismetis]

#193 Introduced template previews.
Now you can navigate to a URL like https://tn-spa-bootstrapper-staging.herokuapp.com/api/template_preview/?template=registration/password_reset.html and see a preview of what that email would look like without having to trigger and receive an email.
This is great, but there could be some improvements:

• If you don't pass it a template name, it should return a list of URLs with all templates it was able to self discover
• The logic for this should probably be in common or utils instead of core

[WajahatKanju]

Could you please offer further elaboration? I'm seeking a more in-depth explanation.

[oudeismetis]

For example, one of the arguments passed to most email templates is support_email, which displays at the bottom of the email. But because of how the template preview feature was built, there was no example support_email to pass. So nothing is displayed in the preview. Thus, the template preview isn't a complete preview of what that email would actually look like.

Example of a template preview in staging here: https://tn-spa-bootstrapper-staging.herokuapp.com/api/template_preview/?template=registration/password_reset.html

This is generated using this template: https://github.com/thinknimble/tn-spa-bootstrapper/blob/main/%7B%7Bcookiecutter.project_slug%7D%7D/server/%7B%7Bcookiecutter.project_slug%7D%7D/core/templates/registration/password_reset.html

Which extends this template where the support_email html code is rendered: https://github.com/thinknimble/tn-spa-bootstrapper/blob/main/%7B%7Bcookiecutter.project_slug%7D%7D/server/%7B%7Bcookiecutter.project_slug%7D%7D/core/templates/_action-email-base.html#L214

[whusterj]

This is getting stale, but leaving it open as a good first issue.