support historic slugs for datasets

## User Story

In order to better support changing slugs for datasets, datagov wants to store historic slugs for datasets

## Acceptance Criteria

[ACs should be clearly demoable/verifiable whenever possible. Try specifying them using [BDD](https://en.wikipedia.org/wiki/Behavior-driven_development#Behavioral_specifications).]

- [ ] GIVEN data providers want to change the title/slug of datasets \
WHEN a slug changes \
THEN the previous one is stored so redirects point from the old to the new \
[AND optionally another verifiable outcome]

## Background
- add a historic slug array

## Security Considerations ([required](https://nvd.nist.gov/800-53/Rev4/control/CM-4))

[comment]: # "Our SSP says 'The Data.gov team ensures security implications are considered as part of the agile requirements refinement process by including a section in the issue template used as a basis for new work.' so please don't remove this section without care."
[Any security concerns that might be implicated in the change. "None" is OK, just be explicit here!]

## Sketch

[Notes or a checklist reflecting our understanding of the selected approach]