Bring back support for custom channel following icons

[advaith1]

Unfortunately, a change was recently made that makes (new) crossposted messages always use the source guild icon, ignoring the configured webhook icon that was previously used. Many servers have custom icons set to better communicate the purpose of that webhook or to distinguish multiple webhooks from the same guild, but now those crossposted messages have a worse experience as they always show the guild's icon. Ideally setting a custom icon would still display that instead of the source guild's icon, or there could be a toggle to use the webhook icon (opting-in to the old behavior).

[ItsRauf]

The way I see it, the source guild's icon should be the default icon. If the guild has a manually set one then it should take priority but if they clear it then it should go back to the default which is the source guild's icon.

[bignutty]

According to the Support Article changing the followed channel webhooks name/avatar is intentional and supported, making this is technically a regression.

[IanMitchell]

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