Export database as OPML file

[Suzie97]

Problem

There is no way to export the current local database and that makes it hard to sync subscriptions, favourites etc. across different machines.

Proposal

Add feature to export the local database as an OPML file. Importing OPML files is already supported.

Alternative Solutions

Using one of the supported syncing services to sync feeds between different systems.

Prior Art

Hubert Figuière has a rust script to export the FeedReader database as an OPML file, https://gitlab.gnome.org/hub/feedreader-export