Harvest-API: `order_by` descending prefix (`-field`) is silently ignored on list endpoints

**Description**
Using a minus prefix to request descending sort order (e.g. order_by=-name) returns the same results as ascending order with no indication the prefix was ignored.

**Steps to Reproduce**
GET https://harvest.data.gov/harvest_sources/?per_page=3&order_by=name GET https://harvest.data.gov/harvest_sources/?per_page=3&order_by=-name

**Expected Behavior**
Minus prefix returns results in descending order.

**Actual Behavior**
Both requests return identical results in the same order.

**Notes**
Confidence: MEDIUM -- tested on /harvest_sources/ with name field only. Needs testing with additional fields and endpoints to confirm the minus prefix is ignored generally.