Private token support for GitHub and npm URLs

[mariusandra]

Changes

• In addition to Github, now private token support is also enabled for GitHub and npm URLs.
• This is necessary to make plugin updates work with github, where we (or the project) will need to enter a global API token in order to not be hit with the 60 requests/min API limit.

Checklist

• All querysets/queries filter by Organization, by Team, and by User
• Django backend tests
• Jest frontend tests
• Cypress end-to-end tests

[mariusandra]

Thanks! Did some slight improvements to the code and the interface:

[mariusandra]

I added two other things and it's ready for a review again.

• GitLab API requests now also use a Header instead of passing the token in the URL query string
• I added global NPM_TOKEN, GITLAB_TOKEN and GITHUB_TOKEN settings. The idea is, at least for cloud, to use a custom GITHUB token in order to get around the 60 requests/hour limit, which makes plugins unusable after a while. Every time we check for a new version or install a plugin, this API is hit, and the 1 requests per minute limit is way too low.
• This could be set per Project/Organization later, but that's scope creep right now.

[Twixes]

Looks good.
Didn't notice any way to extract the tokens out of PostHog. 🤔

[mariusandra]

I think that's a good thing...? 🤔 See also #3082

Thanks for the review! Will fix the conflict and merge.