feat: improve mentions

[Sharp-Eyes]

Description

This PR:

• Implements channel mentions;
• Implements message preview support for mentions in the form of #channel_name, as opposed to only when entered as <#channel_id>;
• Implements mention autocomplete for members in the same sphere;
• Implements mention autocomplete for channels in the same sphere;
• Refactors existing autocompletion logic so all variants can use the same styling and logic;

Note: separate autocompleters do come with their own HTML class so CSS styling can be customised separately.

[cloudflare-workers-and-pages]

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View logs

[EnokiUN]

• not require 2 characters to see channel and user autocomplete
• have proper icon for channels (no channel icons)