[FEAT] remove all stale feature flags

[runspired]

As discussed in the face-to-face, our feature flags have gone very stale and we
we decided to go a different direction on each. This is a debt-collection PR to remove
our stale feature flags and reset the feature-flag clock.

All feature flags that were previously null were removed with their off branch left behind.
All feature flags that were previously true were removed with their on branch left behind.

[runspired]

Starting a list of tickets to update when this merges:

• Ember Data FEATURES aren't merged with Ember FEATURES #5253
• Invalid JSON responses and ajaxOptions depreciation #5338
• Add ability to rollback specific attributes #3705

[jonnii]

@runspired sad about ds-pushpayload-return, it's pretty useful for people who use websockets =(

[runspired]

@jonnii are you saying you are using that feature!?

Are you aware of store.push ?

[jonnii]

@runspired I didn't realize push returned the record it created, you can ignore my comment!

[urbany]

Related: #5265

[sly7-7]

@runspired All green, so I guess this is good to merge ?

Concerning the push-payload thing, store.push does not use the serializer, so if you have an other format coming from the server, you can't directly use store.push. I would to use that feature, but as it is going to be removed, I can do without it (keeping the ids will push, calling pushPayload, then peekRecord on each id). Seems brittle, but at least that only use public api.

[rwjblue]

LGTM...

[runspired]

@sly7-7 an example websocket push that is much less brittle than even the pushPayload use today

// ... payload is raw payload received
let store = this.get('store');
let ModelClass = store.modelFor('foo');
let serializer = store.serializerFor('foo');
let normalized = store.normalizeResponse(store, ModelClass, payload, null, 'query');

return store.push(normalized);