Reset post list on refresh

Summary:

Reset post list on refresh to prevent doubling of if refresh used.

Test Plan:

Throw in a button to refresh the feed (I copied the one that shows on error). Refresh the content and watch it change up several times.

This ensures this can't happen by clearing the list anytime we try to fetch a new set of rss feeds.
Before:

Reviewers: laysrodrigues, patrickelectric, tcanabrava

Reviewed By: patrickelectric, tcanabrava

Subscribers: Atelier

Tags: Atelier

Differential Revision: https://phabricator.kde.org/D17819