Add option to set image_url ("background image") for user

While this option is not used anywhere in microblog.pub itself, some
other servers do occasionally use it when showing remote profiles.

Also, this image _can_ be used in microblog.pub - just add this:

	<img src="{{ local_actor.image_url }}">

in the appropriate place of your template!

app/config.py

@@ -92,6 +92,7 @@ class Config(pydantic.BaseModel):
     summary: str
     https: bool
     icon_url: str
+    image_url: str
     secret: str
     debug: bool = False
     trusted_hosts: list[str] = ["127.0.0.1"]