Catalog API add theme filtering

## User Story

In order to filter on the `theme` field of DCAT-US, catalog users want theme to be filterable/searchable in the catalog API (and possibly on search page).

## 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 the API development is done \
WHEN a request is made to https://catalog.data.gov/search?theme=Transportation \
THEN only datasets with a theme of `Transportation` is returned

## Background

There is guidance for DCAT-US1.1 for Geoplatform to define at least 1 theme of `geospatial` to make it from data.gov to Geoplatform. This guidance is being considered/recommended to continue for DCAT-US3.0.

However, should also be noted many datasets use this `theme` field, and there is interesting content.

## 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

Should be treated similar to `keyword` as an array of strings categorizing data.

Should discuss as a team if worth adding to the UI or just an API implementation.

Should update the documentation if/when complete: https://resources.data.gov/catalog-api/