[2.0] Privacy: "Never Contact External Instances" option

[Sjmarf]

https://lemmy.ml/post/19064688

From 0.19.5 onwards, instance admins can choose to proxy all images through the instance. This is great, because it massively cuts down the number of domains we need to contact to render the feed.

Now that it's within reach, some people might prefer to entirely avoid contacting external instances. We could have an allowlist for instances the user trusts, and/or have an option to pull that allowlist from Fediseer.

Enabling the option would prevent certain features from working. Off the top of my head, you wouldn't be able to:

• See the "Administration" or "Details" tabs in InstanceView
• Search instances
• See the reason in the external API warning sheet

(Some of these could be fixed by proxying those requests through our own server (#1313)).