Split out 'Locations' and 'SequenceReference' into their own tables

[jennifer-bowser]

As part of our storage refactor, we have decided to split out Locations and SequenceReferences into their own tables. See the LucidChart and GitHub Discussion. for reference.

This will require the following:

• Modify functionality when registering variants to store the location and sequencereference in the new tables

• Make the 'vrs_objects' table into an 'Allele' table as part of this. (Copy number variants will be supported in future tickets)

Note that the SequenceReference names should be the refseq aliases from Seqrepo