fix/feat: Restrict proxyAccount to follow a remote user only when the user is not...

What does this PR do? (Please give us a brief description of what this PR does.)

(I'm not sure if this is a feature or a fix, please tell me)

Proxy accounts are used to provide a proxy follow relationship when a user adds a remote user to a list, because due to the mechanism of ActivityPub, if the remote user is not followed, the activity (most likely) will not be transmitted to our server.

But this is not necessary when the user is followed by at least one local user, because the activity will be transmitted to the local server anyway. Follow requests from proxy accounts can be confusing and may not the expected result of the user adding the list.

In this commit, an additional judgment is added for proxy accounts to send follow requests, so that follow requests are only sent when the user is not followed by at least one local user.

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