Skip download in CDN mode

[maiksprenger]

Motivation:

This is a follow-up to #20

#21 added a CDN feature, which means the added script tag will point to the CDN version of mermaid.js. Thank you again for adding that!

But django-mermaid still tries to download a copy to the package's static directory in AppConfig.ready. Because this isn't necessary in CDN mode and causes us a PermissionError during deployment, I'm proposing we only download in non-CDN mode.

All Submissions:

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes to Core Features:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you updated the documentation related to the changes you have made?
• Have you written new tests for your core changes, as applicable?
• Have you successfully run tests with your changes locally?

[maiksprenger]

Thank you for considering, @ArtyomVancyan !

When this is merged, the package will be used improving documentation at a company with nearly a thousand engineers. I think if it's ready for that, it's time to give it a 0.1.0 release, if not even 1.0.0. What do you think? :)

[ArtyomVancyan]

@maiksprenger, thanks for the PR. I totally agree that it shouldn't try to download the assets when it is in CDN mode. Liked the changes in the tests section (separation was a good idea). Also, don't think that 1.0.0 will be appropriate after 0.0.9, so just bumped it to 0.1.0, although 0.0.10 would also be fine as the patch is not big :)

[maiksprenger]

Amazing, thank you for merge and release!