Custom VPN Client - customize the donation reminder

The provider should get the possibility to customize the donation reminder.

The provider should be able (with the help of a build property) to customize the period of time when a reminder is fired again after the last click on the donation link button. The default period of time is one month.

The provider should be able to enable or disable the donation reminder option.

The provider should be able to add custom text resources to the custom flavour that are used as a user message in the donation reminder screen. If no custom text resource are added, a default donation text should be shown.

The provider should be able to set a custom url as a build property that leads to the provider's donation page. If that build property is not defined the LEAP/Bitmask donation url will be used. On click on "donate" in the donation screen a browser should open that URL.