Rename the concept of user-facing "Active" DAG in UI & API about airflow

[kaxil]

Some suggestions from the Dev call on 8th August:

• @dstandish : Unpaused & Paused.
• @bbovenzi : Enabled/paused
• @ashb 's comment: I like pause/unpaused, stale (vs not stale, but not stale doesn’t really show up anywhere) for the states/values in API and DB. (Or paused/enabled is free to if “stale” is the state for dags-where-file-doesn’t-exist anymore)

cc @eladkal

[dstandish]

I think it might be helpful to consider UI and API independently.

First question, what functionality do we want in the UI?

I think I always assumed that it just meant paused vs unpaused.

And, if you don't have orphaned dags, that is acutally true, I think.

Here's a proposal. Just add one more category: "missing" a.k.a. "orphaned" a.k.a. "inactive"

And then you just have 4 categories: all, enabled, disabled, missing

Enabled / disabled seems like better terminology than paused / unpaused. Paused vs unpaused makes less sense for unscheduled dags.