Support variables in the {% now %} template tag

[Lily]

This was discussed and accepted in ​https://github.com/django/new-features/issues/115.

Allow passing a variable from the template context into the {% now %} template tag to allow dynamically choosing the formatting.

​Django's `{% now %}` template tag currently only supports a hardcoded string format defined directly in the template, or one of a small number of specific settings. By supporting template variables, it would be possible to change the format dynamically (for example, based on a user's locale).

​As I explain in my blog post, the now tag has some very weird behaviour caused by assuming the format will always be a string. By expanding to support variables, we can also clean up these weird edge-cases by raising a TemplateSyntaxError.

Several other template tags support variables, so it should be fairly easy to reuse their implementation of this.

[zky]

I have submitted PR #21349 for this feature. Would a triager please update the Triage Stage to Accepted so that the GitHub CI checks can run? Thank you very much!

[zky]

Hello, my previous PR was closed because the ticket hadn't reached the 'Accepted' status. Here is my new PR: ​https://github.com/django/django/pull/21357. Thank you

[zky]

Updated the PR to address the review comments. All checks are passing now. Please take another look, thanks!

[zky]

​https://github.com/django/django/pull/21357