If charm can't connect to external avatar service, it should warn/block

[mthaddon]

Bug Description

Currently if the charm is configured to connect to an external avatar service by an admin in the settings, it needs to be able to reach avatars.discourse.org. If it can't, it should warn about that. Currently there's no indication, and the only way you'd know is that avatars fail to render.

To Reproduce

1. Deploy the charm somewhere that doesn't have access to avatars.discourse.org
2. In the admin interface set "external system avatars enabled" to true, and "external system avatars url" to /letter_avatar_proxy/v4/letter/{first_letter}/{color}/{size}.png
3. Confirm you can't get avatars

Environment

Charmed Kubernetes, Juju 3.1.7

Relevant log output

# From /srv/discourse/app/logs/unicorn.stderr.log
89.64.21.218 - - [16/Apr/2024:16:47:54 +0000] "GET /user_avatar/discourse.ubuntu.com/bandali/144/18097_2.png HTTP/1.1" 503 - 0.0115

Additional context

No response