We need to implement the networking layer of the new Event Platform Client library.
**Interface thoughts**
```
class NetworkManager
init(storageManager: StorageManager)
let storageManager: StorageManager
func httpDownload(url: String, completion: (Data?) -> Void)
func httpPost(url: String, body: String)
func httpTryPost()
```
- `httpDownload` is a simple download method that retries if there's an error, except if response is 404. Log error.
- `httpPost` calls `StorageManager`'s `savePostItem` method
- `httpTryPost` is akin to `EventLoggingService`'s `tryPostEvents`. This will call `StorageManager`'s `fetchPostItemsToPost` and post them. If it succeeded or failed, mark those events via `StorageManager`'s `updatePostItem`.
- `AppViewController` will call `NetworkManager`'s `httpTryPost` as a periodic worker and a background fetcher, like how `EventLoggingService` is set up.
- We need to restore the ability to log an event from anywhere via singleton.
- We need to hook up `EventPlatformClientLibrary`'s `loggingEnabled` up with the permissions in the welcome screen and settings. We also need the ability to reset the session and `appInstallID` when that value is toggled off and back on.
**Known deltas with old library**
`EventLoggingService` posts only on Wifi. It keeps track of when last successful post was made, and if that value gets too large it turns the wifi-only posting off. Should we keep this? Seems like yes.
There's some persisted `lastLoggedSnapshot` logic used in `UserHistoryFunnel`. Doesn't seem like we need this yet.
There's a persisted `appInstallDate` and `loggedDaysInstall` that we use in `WMFDailyStatsLoggingFunnel`. Doesn't seem like we need this.