Update SlimChannelConverter

[z03h]

Rationale

Update SlimChannelConverter to work with mentions and Threads

Summary of changes made

• client.get_channel already resolves threads so a separate thread converter is unneeded
• try guild.get_channel_or_thread first since client.get_channel loops over all guilds
• remove get from message.channel_mentions as it is equivalent to regexing channel mentions from content + guild.get_channel_or_thread and does nothing for messages with no guild

Checklist

• [x] This PR changes the jishaku module/cog codebase
  • [ ] These changes add new functionality to the module/cog
  • [x] These changes fix an issue or bug in the module/cog
  • [x] I have tested that these changes work on a production bot codebase
  • [ ] I have tested these changes against the CI/CD test suite
  • [ ] I have updated the documentation to reflect these changes
• [ ] This PR changes the CI/CD test suite
  • [ ] I have tested my suite changes are well-formed (all tests can be discovered)
  • [ ] These changes adjust existing test cases
  • [ ] These changes add new test cases
• [ ] This PR changes prose (such as the documentation, README or other Markdown/RST documents)
  • [ ] I have proofread my changes for grammar and spelling issues
  • [ ] I have tested that any changes regarding Markdown/RST syntax result in a well formed document

[Gorialis]

Note to self that I need to make SlimThreadConverter an alias of this converter so that it doesn't break anything that uses it