Move adding feature flags to event to capture endpoint

[Twixes]

Changes

This will save us a ton of work by moving adding feature flags to event properties earlier in the Python part of the pipeline, instead of forcing us to redo this in JS.

[timgl]

Fine to merge, but can you keep an eye on the endpoint stats to see if this dramatically increases the time it takes to post stuff?

[mariusandra]

This changes one thing with all events: adds a properties object. Until now many plugins had hooks like if (event.properties) { event.properties.hello = 'world' }, as some events, namely $identify don't come with properties.

[mariusandra]

Also, tests are failing because of this.

Note, I'd actually be in favor of making sure all events always have a properties dict attached to them. It would make a lot of plugins easier to write, as you wouldn't have to worry if the object is undefined or not.

[Twixes]

Obvs, will watch this @timgl
Yep, noticed @mariusandra Actually I just made the change so that it'd only add properties if also filling it with $feature_flags to conform with tests, but we can also ensure that properties are always a thing. WDYT?

[mariusandra]

I guess we can accept missing properties here and just add an empty object in the plugin server to make life easier for plugins.