Ask the user to populate feed on add.

[jtagcat]

Current state: Nothing happens when a feed is added. Every time the feed is refreshed, depending on the config, 2 items get posted to the channel, until all old items are posted.

Proposed:

1. After feed is added, ask the user for yes/no (buttons!) whether to get existing items, or only new items from the current moment forward (mark all items in feed 'posted').
2. If posting existing items, post all at once.