Fixes for activities endpoints.

[svogt0511]

Purpose

Should fix a several problems.

1. The first is the NoMethodError: undefined method `name' for nil:NilClass #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.
2. Added a fix for Unable to retrieve single Activity records via the REST API #1060 where the /activities/:uid endpoint would always return not found. Turns out it was not implemented.
3. This is the same as item 1, described, above: activities endpoints may return a 400 error #1062
4. Minor fix for the dev environment setup so activities can successfully be tested manually: Fix for dev environment ES index setup for activities #1065
5. 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:

closes: #1054, #1060, #1062, #1065

Approach

Open Questions and Pre-Merge TODOs

Learning

Types of changes

• 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)

Reviewer, please remember our guidelines:

• Be humble in the language and feedback you give, ask don't tell.
• Consider using positive language as opposed to neutral when offering feedback. This is to avoid the negative bias that can occur with neutral language appearing negative.
• Offer suggestions on how to improve code e.g. simplification or expanding clarity.
• Ensure you give reasons for the changes you are proposing.

[digitaldogsbody]

A few minor things but non-blocking

[ashwinisukale]

Changes look good, just suggested some refactoring changes