fix(plex): a link too slow for the stream resumes in place, and says so

A request hls.js got no answer to in time (no status) was taken for a server
away: a new session, the same first segment lost again, a restart every half
minute that never ended, since every session answer reset the away window.
Measured on prod: a work network at ~4 Mbit/s against 11-16 MB remux segments.

A timeout now resumes loading in the same session; the away window ends only
when media arrives (or a direct stream answers), and runs out in a 'too slow'
error. A segment gets 30 s to its first byte. After 5 s of stalling on a link
measured slower than the stream, the player says so with both rates.

frontend/src/i18n/locales/en/plex.json

@@ -191,6 +191,9 @@
     "errNotFound": "This file can't be played — the media isn't reachable on the server (the Plex media mount is missing or the path map is wrong). Ask the admin to check the Plex configuration.",
     "errTranscode": "This file's format needs transcoding, which isn't supported yet.",
     "errGeneric": "Playback couldn't start. Please try again.",
+    "errSlow": "Couldn't play: the connection is too slow for this file",
+    "slowLink": "Slow connection",
+    "slowLinkRates": "This file needs about {{need}}, about {{got}} is arriving",
     "help": "Keyboard shortcuts",
     "infoBtn": "Media info",
     "more": "More",