* Fix muting of feeds gone with HTTP 410
A feed answering HTTP 410 Gone is meant to be muted (#3561), but:
* Since #6037, FreshRSS_Feed_Exception extends Minz_Exception, which
resets any code other than its own error levels to 0, so the check for
410 never matched. FreshRSS_Feed_Exception now keeps the HTTP status
code it is given.
* FreshRSS_FeedDAO::mute() mutes by negating the TTL, which leaves a feed
with the default TTL (0) unmuted. Mute the feed the way the feed
settings and dynamic OPML categories do instead.
* A bit of cleaning
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Co-authored-by: Alexandre Alapetite <alexandre@alapetite.fr>
