Fixes: https://github.com/jitsi/jitsi-meet/issues/6665

The welcomePageEnabled and pictureInPictureEnabled props on mobile have been
converted to feature flags.

- handle some weird bug
(https://github.com/firebase/firebase-ios-sdk/issues/233)
- use a common function to extract the URL off a dynamic link

The SDK will now search for an asset called "CallKitIcon" on the main bundle,
and fallback to a built-in asset it it's not there, allowing SDK users to
customize it by just adding asset with that name.

It complicates automated builds.

Consolidate all failure cases into a single one: CONFERENCE_TERMINATED. If the
conference ended gracefully no error indicator will be present, otherwise there
will be.

Since the SDK may be embedded with other apps, we need to recognize our custom
URL scheme and universal links in order to tell the user if we will process the
request or not.

Make them configurable with sane defaults.

- cleanup
- API simplification (single loadURL method)
- JitsiMeet singleton for linking helpers and globals
- Linking moved to app

We are going to implement the invite dialog *inside* the SDK, so there is no
need to have all this machinery anymore.

This marks our switch to CalVer: http://calver.org/

Major: year
Minor: release number
Patch: build (in case we need to retry)

No caps are allowed, so this would trigger a rejection when uploading to
TestFlight.

This is mostly implemented in the app, with the needed support in the SDK. Since
the app needs to donate intents and deal with creating NSUserActivity objects it
doesn't feel right to do this in a library. Instead, we donate the intents from
the app, but the SDK is ready to extract conference URLs from any intent which
was registered as a conference activity.

This also opens the door for eventually adding Handoff support.

This is done at the app level, not the SDK.

Currently 2 Firebase services are used:

  - Crashlytics
  - Dynamic Links

They are enabled in tandem, if the appropriate Google services file
(GoogleService-Info.plist on iOS or google-services.json on Android) is found.

Each service needs to be individually enabled in the Firebase console.

It doesn't add any value, by default HTTP is not supported unless an exception
is added, plus it doesn't work in browsers at all.

WebRTC no longer actively fixes iOS 9 issues.

If the beginAddPeople delegate method is implemented, and they haven't been
explicitly set to NO, they willbe YES.