Add support for iCloud Drive

[markseltenrijch]

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User feedback

I was wondering if you were planning on adding iCloud Drive support so that filter lists are saved across devices. What I like best about your app is the unlimited number of subreddits I can block from All, but whenever I get a new device adding everything back can be time-consuming.

https://secure.helpscout.net/conversation/513279347/1890?folderId=356571

Device information

Device model: iPhone8,1
iOS version: 11.2.5
App version: 2.6.1