Appservice Can't set alias

*This issue was originally created by [**@nkitsaini**](https://github.com/nkitsaini) at .*

Same as #661 which is closed but I can still reproduce the issue.

### Background information

- dendrite v0.5
- Monolith
- Sqlite3
- In Docker

### Description

Appservice can't set alias in it's own namespace.
Request fails with error message: `Alias is reserved by an application service`

### Steps to reproduce

- Start Dendrite with a appservice
- Try to set alias using appservice

### Logs
Can't find anything relevant in debug logs.