Add basic backend validation for selectLinks

[tw4l]

trafficstars

Follow-up to #2152

Related to https://github.com/webrecorder/browsertrix/pull/2487

This PR provides very basic validation of the config.selectLinks argument on workflow creation and update. Namely, it checks that:

• config.selectLinks is not an empty array
• Each entry consists of two non-empty text sequences separated by ->

At this point we're not validating the actual CSS selector on the backend, though we could add that down the road.

Tests have been added accordingly.