Change "Content Warning" to "Content Notice", remove warning iconography

Content warnings on Mastodon are being used (or maybe always were) as more of a "spoiler text" or something to denote what the content is. This isn't just done for the typical "Content Warning" (food, eye contact, etc) but also for easy filtering - like politics, personal life talk and more. This isn't really deserving of the warning iconography we give it, and more people are against that in upstream Mastodon as well.

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