[commands] Fix clean_content converter not properly escaping mentions.
[commands] Fix clean_content converter not properly escaping mentions.

In some cases, the Discord provided role_mentions and mentions array
would be empty, such as wrapping a mention with a backtick or
other frivolous characters. Since we want to completely nullify
mentions, we should not rely on the Discord provided arrays and instead
use and resolve the IDs from the content itself.

imayhaveborkedit/discord.py,Harmon758/discord.py,khazhyk/discord.py,Rapptz/discord.py,rapptz/discord.py,Harmon758/discord.py