Add ability to expand all content warnings on posts

[BlankEclair]

Summary

When viewing a post with a content warning, its replies will typically also have the content warning, so the user will have to expand all of them at once:

Purpose

On Sharkey (and probably Misskey), when viewing a post, there's a button that automatically shows the contents of all posts:

Do you want to implement this feature yourself?

• Yes, I will implement this by myself and send a pull request