Tighten up terms for "label" vs. "category"

[nathanielrindlaub]

We use both throughout the app - we should come up with specific definitions and make sure we are using them consistently.

Related: it would be worth tightening up our verb-usage around "creating" or "applying" a label now that we have Image.labels and Project.labels. Right now,createLabels technically applies a label to an Image record, while createProjectLabel creates a new project label. Maybe it's clear enough as is?