[feat] Update check (#4620)

The concept is quite simple: stick a file on the OTA server named
something like `koreader-appimage-latest-stable` (by analogy with
`koreader-cervantes-latest-stable.zsync`), which contains nothing
but a filename.

The difference with the zsync update is that the link is then launched
in the user's browser (AppImage) or DownloadManager (Android, not yet
implemented).

* Android hasOTAUpdate = no for the moment

frontend/device/android/device.lua

@@ -21,7 +21,7 @@ local Device = Generic:new{
     display_dpi = android.lib.AConfiguration_getDensity(android.app.config),
     hasClipboard = yes,
     hasColorScreen = yes,
-    hasOTAUpdates = yes,
+    hasOTAUpdates = no,
 }
 
 function Device:init()