Fixes for activities endpoints.

Purpose

Should fix a several problems.

The first is the https://github.com/datacite/lupo/issues/1054, where trying to retrieve activities for a DOI would cause a 'NoMethodError: undefined method 'name' for nil:NilClass' error when trying to display activities using /dois/:id/activities endpoint older DOIs in production. This was a problem in the serializer.
Added a fix for https://github.com/datacite/lupo/issues/1060 where the /activities/:uid endpoint would always return not found. Turns out it was not implemented.
This is the same as item 1, described, above: https://github.com/datacite/lupo/issues/1062
Minor fix for the dev environment setup so activities can successfully be tested manually: https://github.com/datacite/lupo/issues/1065
Also found it necessary in the activities serializer to guard to guard against audited changes that show an initial value of null for publisher or publisher_obj. This happens when these fields are first assigned a value. An example be found in the audit date for https://api.datacite.org/activities/7b333de3-77af-4200-8ffd-c8b2012a8d9e:

[x] Bug fix (non-breaking change which fixes an issue)
[ ] New feature (non-breaking change which adds functionality)
[ ] Breaking change (fix or feature that would cause existing functionality to change)

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