Refactor hardcoded JS CDN links to _protected variables

[and-viceversa]

folium.py exposes _default_js and _default_css at a high level. This is good, because you can override the CDN URL to point elsewhere. In my case, to point to a static assets folder.

The plugins do not do this. This PR moves the URLs out of their respective render() methods to expose them in a similar way. This shouldn't change any existing behavior.

Also, 98% of the changes are fixing flake8 results. Sorry about that but I went by the contributing guideline.

[Conengmo]

Hi @and-viceversa, thanks for your PR, I think this is a good idea!

For anyone reading along, this was discussed in #351 and more recently in #1312.

Two things:

I'd prefer if each module had the same variable name for the JS and css links. Since we already have _default_js and _default_css in folium.py we can use those names. And also have them all be the same type, a list of str.

Can you remove the style changes in code you otherwise wouldn't touch? It makes reviewing a lot more work, and since I'm doing this in my free time that's something to look out for. I'll update the contributing guide to reflect this as well.

[and-viceversa]

Please see updated PR.

The plugins that use them now follow the _default_js and _default_css pattern.

The render() methods use the same for loop as folium.py.

Most plugins only use 1 JS or CSS file, so this is a bit overkill. But now it's uniform.

Thanks for your quick feedback. Let me know what else I can do on this.

Edit: Also, if the PR looks like Git spaghetti - first time doing this not on my own repos. Please advise if that part needs to be cleaner.

[Conengmo]

Looks good @and-viceversa ! I'll check in more detail before merging, but at this time there's nothing else for you to do. Don't worry about the commits, we squash and merge.

[Conengmo]

Thanks for your work on this @and-viceversa !