Feature request: delete all existing feeds at import

[scroggi]

I use the feed import function to keep sync between Miniflux and NetNewsWire, which I use more frequently.

While Miniflux will ignore existing feeds from an import, this only works in an additive fashion and mass removal of feeds I am no longer reading is time consuming since you have to delete them one at a time from the feeds page.

It would be nice to have an option to delete all existing feeds when importing a new OPML. This would allow clean baselining against an OPML file at import.