api: Properly parse invite events

Events are wrapped in a content block like so:

```
"content" : {
    "membership" : "invite"
}
```

However we were parsing it as just `"membership" : "invite"`.
Now we properly expect it to be within a content block.

Fixes GNOME/fractal#456
4 jobs for wip/christopherdavis/fix-invites in 14 minutes and 26 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Check
passed #287260
check

00:00:28

 
  Test
passed #287261
flatpak

00:13:01

 
  Review
passed #287262
review

00:00:43

failed #287263
allowed to fail manual
stop_review

00:00:12

 
Name Stage Failure
failed
stop_review Review
Created fresh repository.
From https://gitlab.gnome.org/BrainBlasted/fractal
* [new branch] master -> origin/master
* [new branch] room_member_events -> origin/room_member_events
* [new branch] wip/christopherdavis/fit-room-directory -> origin/wip/christopherdavis/fit-room-directory
* [new tag] 0.1.0 -> 0.1.0
Checking out 2018a70c as wip/christopherdavis/fix-invites...
fatal: reference is not a tree: 2018a70c752dc01b2fd97ab109b56f9b9434aac2
ERROR: Job failed: exit code 1