This commit adds initial support for CallKit on supported platforms: iOS >= 10.

Since the call flow in Jitsi Meet is basically making outgoing calls, only
outgoing call support is currently handled via CallKit.

Features:
 - "Green bar" when in a call.
 - Native CallKit view when tapping on the call label on the lock screen.
 - Support for audio muting from the native CallKit view.
 - Support for recent calls (audio-only calls logged as Audio calls, others show
   as Video calls).
 - Call display name is room name.
 - Graceful downgrade on systems without CallKit support.

Limitations:
 - Native CallKit view cannot be shown for audio-only calls (this is a CallKit
   limitaion).
 - The video button in the CallKit view will start a new video call to the same
   room, and terminate the previous one.
 - No support for call hold.

It provides access to the app's display name and version.

Before, Jitsi Meet (the app) would only link with JitsiMeet.framework, which in
turn embedded WebRTC.framework. While possible, Apple doesn't allow apps with
nested frameworks to be submitted to the store. Now the app will link with
WebRTC.framework directly so there is no framework nesting.

A potential improvement here is to build WebRTC as a static library so it can
then be embedded in JitsiMeet.framework and completely hidden from the app.

The current implementation doesn't use the API and Transport modules. This is
due to the fact that they are too tied to APP at the moment, which is web only.

Once API is refactored and moved into the Redux store this will be adjusted,
though it's unlikely that the lowest level React Native module (ExternalAPI)
changes drastically.

This commit also introduces a stopgap limitation of only allowing a single
instance for JitsiMeetView objects on both Android and iOS. React Native doesn't
really play well with having multiple instances of the same modules on the same
bridge, since they behave a bit like singletons. Even if we were to use multiple
bridges, some features depend on system-level global state, such as the
AVAudioSession mode or Android's immersive mode. Further attempts will be made
at lifting this limitation in the future, though.

1. Aligns the project structure of Jitsi Meet SDK for iOS with that for
   Android for better comprehension.

2. The command `react-native run-ios` uses the last Xcode project or
   workspace in the list of these sorted in alphabetical order. Which
   limits our freedom in naming. Thus having only an Xcode project in
   the root directory of the iOS project structure gives us back the
   freedom in naming.

3. Allows the Podspec to work for the app project in addition to the sdk
   project because we need Crashlytics in the app which is integrated
   via Cocoapods as well.

4. Further removes references to JitsiKit in the source code for the
   sake of consistent naming.

Ladies and gentlemen, allow me to introduce you to Jitsi Meet SDK for iOS, the
mobile SDK which powers Jitsi Meet.

The goal is to encapsulate the entire React Native app into a framework / SDK
and offer an API for native (ObjC or Swift) applications to embed the Jitsi
conferencing experience.

While React Native can be embedded in native applications, I don't think it was
designed to be embedded as part of a framework, hidden away from the application
using it. This surfaced as a number of issues which had to be addressed
specifically due to our use-case:

- Universal / deep linking needed to be wrapped to avoid the embedding app from
  linking with RN.
- The bundle URL had to be manually constructed, since RN considers that all
  resources are in the main bundle, but in case of a framework that is not the
  case.
- Custom fonts had to be manually loaded, since UIAppFonts doesn't work on the
  framework's Info.plist file.
- The RN packager has to be manually triggered since the React project will no
  longer do it for us.
- Custom App Transport Security rules were added since the builtin way to do it
  modifies the framework's Info.plist, which is useless in this case.

At this stage, the Jitsi Meet application is just a small single view
application which uses the Jitsi Meet SDK to create a single view which
represents the entire application. Events and external conference handling are
forthcoming.