Prosody Appimage: supports only x86_64:

* stop trying to generated an ARM64 AppImage (as it is buggy)
* display a warning in the settings if incompatible CPU detected
* documentation for a workaround

Implements #120.

server/lib/prosody/config/paths.ts

@@ -8,9 +8,9 @@ interface ProsodyFilePaths {
   certs: string
   modules: string
   avatars: string
-  exec: string
+  exec?: string
   execArgs: string[]
-  execCtl: string
+  execCtl?: string
   execCtlArgs: string[]
   appImageToExtract?: string
   appImageExtractPath: string