Capture harvest source descriptions

## User Story

In order to capture harvest source subscriptions, data.gov admins (and data providers) want the H2.0 model updated to allow for a description.

## Acceptance Criteria

- [ ] GIVEN the Harvest 2.0 model is updated \
WHEN a sync between CKAN and H2.0 occurs \
THEN the description (where available) is available on H2.0.

## Background

See prod example here: https://catalog.data.gov/harvest/about/census-5-digit-zip-code-tabulation-area-zcta5-national

Ironically, the description is cut off in the view. However, as we lack context on many of these harvest sources, it will be helpful to have this information for future.

## 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."
None

## Sketch

This would involve a DB migration, but only adding a field (which is minor by DB migration standards).

- Add `description` to the harvest_source table model via a migration.
- Validate working, and release through dev/stage/prod.
- Write script to [download all the descriptions](https://catalog.data.gov/api/action/package_search?fq=dataset_type:harvest&fl=title,name,notes&rows=1000) and update the corresponding harvest source in H2.0. It should be assumed (unless proven otherwise) that the name is the matching field between the 2 systems.
- Validate/update the harvest source page (UI) to show the description.