Add queue mechanism for importing new projects.

[scovetta]

We need a way for users (permissioning TBD) to add new projects to the metric dashboard. This should be import via URL or PackageURL.

Option A: We should add these to a queue, and have a job that drains that queue. We already have the import code in management/commands/import_project.py, so this feature should be relatively straightforward. We're going to defer importing additional types until Microsoft/OSSGadget#152 is completed, and then just call out to that, unless someone objects to the hard dependency there.

Option B: Move the job to the analysis container, and make "importing a project" just another type of analysis. This keeps coupling loose and should let us scale out as needed.

We still need to think more about this.

[bnkamalesh]

I think we need to setup contribution guidelines for this project itself, might turn out to be a mess if we get enough contributions.