End to End encrypted Push Notifications

Closes #15 (closed)

Depends on foodsharing!1647 (merged)

What does this MR do?

Adds push notitifications for new chat messages

• Notifications are encrypted using https://github.com/google/capillary (based on the web push protocol)
• Messages are displayed using the MessageStyle (e.g. messages from the same conversation are grouped)
• Quick Reply functionality in the notification
• Adds a settings menu (based on @alex.simm branch) which allows to enable / disable notifications
• A banner is shown after the first start prompting users to enable push notifications
• Adds a play and foss build flavour, the latter allows to build the app without depending on google play services (e.g. to publish it on f-droid)

Links to related issues

#15 (closed)

Checklist

• no unrelated changes
• joined #fs-dev-android-beta channel at https://slackin.yunity.org
• added an entry to CHANGELOG.md