As a user, I can upload modular content

[pulpbot]

Author: @dralley (dalley)

Redmine Issue: 6271, https://pulp.plan.io/issues/6271

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A user should be able to upload a module metadata YAML file to Pulp, which will create modulemd and modulemd-default content units.

When new modules are created, they should be automatically associated with any referenced RPMs already present or which are added in the future.

In case modulemd-deafults, there is restriction that only one modulemd-defaults can exist for a repository, so we will either need to have a total replacement strategy, a merge strategy like with Errata, or require the user to delete the existing ones first. Validation will be very important.