Create a class for MediafluxRootCollectionAsset

[bess]

User story

A MediafluxRootCollectionAsset is a top level asset that all collections get created into. It's not a Project collection, it is the root of the Mediaflux system. Each project would be a "branch" off of this.

Acceptance criteria

• [ ] Calling MediafluxRootCollectionAsset.create! should ensure the root/trunk asset exists, and create it if it does not
• [ ] asset_create_request.rb is the class that currently does this, but it doesn't have a #create! method
• [ ] The #create! method should be idempotent -- if it exists already, do not recreate it

Implementation notes, if any

[bess]

Hey team! Please add your planning poker estimate with Zenhub @carolyncole @hectorcorrea @JaymeeH @jrgriffiniii @leefaisonr