RSS Guard

Fixed:
* MariaDB no longer creates duplicate articles when feeds return more than ten items. (#2410)
* Opening a file in an already running RSS Guard instance no longer crashes after closing feed discovery. (#2407)
* Added a couple of fixes in relation to issue #2408:
* Made UI texts more consistent when talking about "attachments", also fixed in documentation.
* Changed UI text for "Open articles URL directly..." feature and added help spoiler and tooltip explaining its function to it.
* "Play in media player" feature is now expanded and it offers dropdown submenu where user can "play" article attachments. Same behavior was expanded in article list context menu.