Fix bug where @everyone usernames bypass Message.clean_content.
Fix bug where @everyone usernames bypass Message.clean_content.

Rapptz/discord.py,variablehair/Eggplantato,khazhyk/discord.py,Harmon758/discord.py,jhgg/discord.py,imayhaveborkedit/discord.py,lgkern/PriestPy,Harmon758/discord.py,Theoretical/UselessBot,rapptz/discord.py,mgardne8/discord.py