[feature/frontend] Add options to include Unlisted posts or hide all posts

[tsmethurst]

Description

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This pull request adds a new account setting that determines the visibility of posts that will be included in an account's web view and RSS feeds.

Closes #3249
Closes #900

Checklist

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• I/we have read the GoToSocial contribution guidelines.
• I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
• I/we have not leveraged AI to create the proposed changes.
• I/we have performed a self-review of added code.
• I/we have written code that is legible and maintainable by others.
• I/we have commented the added code, particularly in hard-to-understand areas.
• I/we have made any necessary changes to documentation.
• I/we have added tests that cover new code.
• I/we have run tests and they pass locally with the changes.
• I/we have run go fmt ./... and golangci-lint run.

[NyaaaWhatsUpDoc]

the way this is worded, this makes it sound like the default behaviour is that we allow unlisted posts to be shown in the web view. i'd maybe reword the beginning to say something along the lines of "if you haven't already (this is the default behaviour)"