Activity subject fixes

[acoffman]

When Variant Coordinates were accepted, the subject was set as the underlying coordinate models, allowing that fact to leak through to the users.

Now, instead of setting the activity subject to the revision's subject, it will be set to the revision's creation activity's subject (which will be the variant).

Removes VariantCoordinate and ExonCoordinate as valid subject types.

We will need to run the following backfill script on deploy:

a = Activity.where(subject_type: ['ExonCoordinate', 'VariantCoordinate']).all
a.each { |x| x.subject = x.linked_revisions.first.creation_activity.subject; x.save! }