Allow projects to be imported from other sites

[jasonaowen]

Allow a user to create a new project by importing a project from a canned hosting site like GitHub, GitLab, or Bitbucket.

Data to be imported could include the project title, authors (at least on GitHub, as we have Recurser's GitHub username through the RC API), and description. We could also add the repo link and the GitHub website link, either as Markdown (#23) or as first-class links (#21). Once we add tags (#5), we can also import GitHub topics, in addition to language tags inferred from languages used in the repository. Presumably there are some GitLab and Bitbucket-specific things we could import, as well.

There is some question of whether or how to keep the portfolio project in sync with the place it was imported from. Would we want to automatically update all the imported things? If we add additional import functionality, should that automatically apply to previously imported projects? If no, can users opt into newly added functionality later?

Thanks to @blinry for suggesting this.