Send impersonated user ID for appservice login

[AndrewFerr]

Otherwise, all /login calls would target the main appservice user.

Signed-off-by: Andrew Ferrazzutti andrewf@element.io

Checklist

• [ ] Tests written for all new code
• [x] Linter has been satisfied
• [x] Sign-off given on the changes (see CONTRIBUTING.md)

[AndrewFerr]

This is a follow-up of #275.