Allow for the GET /listings request param to specify how the results should be ordered

[anders-schneider]

Issue

• Addresses Add ability to sort listings to backend #193

Description

This change adds an orderBy field to the ListingsQueryParams so that requests to GET /listings can indicate how the listings should be sorted. Currently there are only two options: order by most recently updated or by "application dates".

("application dates" is the pre-existing behavior: order by soonest applicationDueDate, and then by earliest applicationOpenDate. Since "application dates" was the pre-existing behavior, this change preserves that as the default ordering if no orderBy param is specified.)

A few other smaller notes:

• The pre-existing behavior also sorted units within a listing by max_occupancy. This change keeps that behavior, by applying the max_occupancy sorting on the full query. (There's no need to apply that on the innerFilteredQuery because it doesn't return individual units.)
• This change updates sites/public so that the full listings page requests listings with the orderBy=mostRecentlyUpdated parameter.
• This change updates some of the seed listings to have "more interesting" application dates.

Type of change

• 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 not work as expected)
• Prototype/POC (not to merge)
• This change is a refactor/address technical debt
• This change requires a documentation update
• This change requires a SQL Script

How Can This Be Tested/Reviewed?

• Desktop View
• Mobile View
• $ cd backend/core && yarn test
• $ cd backend/core && yarn test:e2e:local

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have reviewed the changes in a desktop view
• I have reviewed the changes in a mobile view
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have assigned reviewers
• I have updated the changelog to include a description of my changes
• I have run yarn generate:client if I made backend changes

[avaleske]

LGTM