[feature] Ratelimit + serve emoji images on separate router group

[tsmethurst]

Description

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This pull request serves the emojis part of the fileserver under a separate router group, applying separate rate-limiting (2 x configured rate limit). This should help with cases where someone has uploaded a few hundred custom emojis, and ends up getting rate-limited when using the emoji picker (which loads all emoji images at once, basically).

Also adds a warning to the emoji upload page recommending admins limit emoji use to ~ 300-400 custom emojis.

closes #2537

Checklist

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• I/we have performed a self-review of added code.
• [x ] 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.
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• I/we have added tests that cover new code.
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