🦉 allowing external images as setting

For the config.json (server-level default for new users):

```json
{
"defaultSettings": {
"externalImages": "homeserver"
}
}
```

User's own choice in the UI always overrides the config.json default.

owl/README.md

@@ -19,3 +19,8 @@ In this owl folder should go all documents we produce for this Endeavour.
  - Find out how we can set a theme by default for new users.
  - img tags in messages should be rendered, without requiring them to be on a local cache. We do not need to secure against identification attacks.
  - youtube links should create embedded players (if enabled).
+
+ 
+ ## Commit Messages
+ 
+ All Commits for OWL changes start with the Owl Emoji 🦉