fix(converter): fix clean_content not using nicknames in some cases

[shiftinv]

Summary

Tiny bug I noticed the other day - in the clean_content converter, user objects always took precedence over members, which resulted in usernames being used instead of nicknames.
This only happens when the user ID isn't part of ctx.message.mentions, which is the general case with interactions.

To reproduce:

# with members intent enabled, invoke `/test arg:@mention`
@bot.slash_command()
async def test(inter: disnake.CommandInteraction, arg: str) -> None:
    await inter.send(await commands.clean_content().convert(inter, arg))

Checklist

• If code changes were made, then they have been tested
  • I have updated the documentation to reflect the changes
  • I have formatted the code properly by running pdm lint
  • I have type-checked the code by running pdm pyright
• This PR fixes an issue
• This PR adds something new (e.g. new method or parameters)
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, ...)