Add support for --template argument to wagtail start. Fix #10423

[thibaudcolas]

Fixes #10423. This is the "MVP" implementation which makes it possible to pass a --template to startproject, without any other niceties.

Potential improvements once we have a clearer idea of what to do with this:

• Documentation. Currently there are only a few mentions of the wagtail CLI, no single source of truth.
• Support for aliases / blessed template sources (Support for custom templates in wagtail start command #10423 (comment)). This would need building on top of startproject.
• Support for custom extensions, name, and other startproject options.
• Perhaps nicer CLI feedback or support for more startproject options
• Unit tests? I couldn’t see any for start nor updatemodulepaths

---

• Do the tests still pass?[^1]
• Does the code comply with the style guide?
• [ ] For Python changes: Have you added tests to cover the new/fixed behaviour? No, there are no tests for this code currently.
• [ ] For front-end changes: Did you test on all of Wagtail’s supported environments?[^2]
• [ ] For new features: Has the documentation been updated accordingly? No, there are no docs for this CLI

To test this, make sure start still works with the default, and make sure it works with a --template:

wagtail start mysite
wagtail start mysite --template=https://github.com/thibaudcolas/wagtail-tutorial-template/archive/main.zip
# Potentially compare to:
./wagtail/test/manage.py startproject mysite --template=https://github.com/thibaudcolas/wagtail-tutorial-template/archive/main.zip

[squash-labs]

Manage this branch in Squash

Test this branch here: https://thibaudcolasfeature10423-start-lca3z.squash.io

[laymonage]

Tested with the provided examples, and also by downloading the template and passing the local file path. Yeah I'm not too sure about unit tests too.

Passing an incorrect template URL or file path seems to throw a descriptive error too (which I assume comes from Django's command).

Awesome work, thank you very much!

[laymonage]

I suppose we could also do something like __file__.parent().parent(), but I'm not sure that's a better approach in case we move this file around

(Just noticed: this is also in the original implementation, so all good!)

My suggestion would be to extract the logic to a separate method, e.g. get_default_template() maybe?