[Starboard] Configuring allowed role prevents starring messages from any channel

[Injabie3]

Describe the bug
After applying the fix for #261 and #266 (via #267), it appears that it prevents any channel from starring any messages when there is an allowed role configured.

This is because when evaluating the allowlist, we call function check_channel. It checks for self.whitelist, which contains both channel IDs and role IDs, and because there is a whitelisted role, it assumes that there are whitelisted channels as well, which there aren't, and returns False. When if any(..., not allowed_channel) is evaluated, any(..., not False ) evaluates to True and we don't highlight the message.

To Reproduce
Steps to reproduce the behavior:

1. Configure an allowed role.
2. Configure a denied channel (should be optional)
3. Have people with an allowed role star a message in the any non-denied channel.
4. Notice that message isn't starred.

Expected behavior
The message to be highlighted.

Screenshots
(Similar screenshot from #261)