Persistence preset: RSS feeds

Originally created by @intrigeri on #7625 (Redmine)

It would be good to make it easy to subscribe to RSS feeds, and have the list of subscriptions and status of feeds items (read, unread, etc.) persistent.

The most likely option seems to use Icedove to provide this feature. If it works out well, then we can drop Liferea. Else, we’ll have to either:

• investigate other options (e.g. the Firefox Sage addon
• or go back to trying to use Liferea for this, and fix the issues identified when we tried it the first time (#5711 (closed) and subtasks)

Related issues

• Related to #5711 (closed)
• Related to #7626 (closed)
• Is duplicate of #15775 (closed)