Rules: set up localised push notifications in When-Then rule

[DonWillems]

As a application manager I want to set multiple languages for the push notifications that are sent from When-Then rules, so that end users receive the notification in their preferred language.

• Extend When-Then push notification message UI to allow for setting multiple languages, and setting which is default
• Set the user locale attribute based on the language they set in the webapp. In some apps this is initially set automatically by browser language, and can be overwritten by the user. Use this language to write to the user locale (a user attribute?)
• When the notification is sent, the rule should check the local of the user and then sent that notification.

Design will be added.

Later:

• Apply to email notifications

[DonWillems]

Duplicate of #1401