Package Revival - Firebase Cloud Messaging support

[fishsaidno]

This PR is based on #354 which was never merged. It's been manually rebased against the current master branch and updated for current-day compatibility.

The docs have been given a bit of a freshen up (I couldn't spend too much time here, they are still outdated in places).
Please review the documentation changes in the commit history for the full picture.

Notable changelog:

• FCM Firebase Cloud Messaging support for both Android & iOS. New token type { fcm: }

• BREAKING CHANGE: Meteor v2.14 is required (for Cordova compatibility)

• BREAKING CHANGE: Push.Configure on the client now takes a cordovaOptions argument for configuration which is passed directly to cordova-plugin-push

• BREAKING CHANGE: The Push.send argument android_channel_id has been renamed androidChannelId for consistency with other package argument names

• Swapped out phonegap-plugin-push (deprecated) for @havesource/cordova-plugin-push

• Added direct support for NotificationPriority / NotificationInterruptionLevel for Android and iOS respectively when sending via FCM, with the androidPriority and iosPriority arguments

• Bumped cordova-plugin-device to v2.1.0 (previously 2.0.2)

This branch is currently in use in a production application, and I will be maintaining it in that respect for at least the next 2 years.

What needs to be done:

• I'm not entirely sure, but it seems like the Community Packages team may not have access to the raix:push package on Atmosphere? This will prevent a new release being published. Can install as a local package or git submodule until that's resolved.
• When merged, versioning in this package needs to be addressed. This release ought to be published as v5.0.0 (major). I propose to do away with automatic versioning via semantic-release-meteor and manage assigning the version numbers manually.
• You may need to force the raix:eventstate version to 0.0.5 in your app's packages file. The latest published version has had all the code stripped out and just throws an error. That package ought to be un-deprecated and a new release published with versions 0.0.5's code restored.
• There's an upstream bug regarding background notifications on Android. See docs/PROBLEMS.md for more.