feat: add optional password-manager-service plug

A test that might address #300, due to a suggestion
from the upstream that the issue is access to a stored
token in the user's keyring.

Will not autoconnect, and users will need to:

    snap connect signal-desktop:password-manager-service

Should be reverted if it does not address the issue.

snap/snapcraft.yaml

@@ -190,6 +190,7 @@ apps:
       - removable-media
       - unity7
       - screen-inhibit-control
+      - password-manager-service
     environment:
       TMPDIR: $XDG_RUNTIME_DIR
       # Included temporarily until https://github.com/snapcore/snapcraft-desktop-integration/issues/28