title: discord.py on_reaction event not detecting messages after the bot is run
body: <p>Whenever we add a reaction to a message that was sent before the bot has started, it goes undetected. Any fixes?</p>
  <p>Code: (doesn't really matter to the question)</p>
  <pre><code>@client.event
  async def on_reaction_add(reaction, user):
    if not user.bot:
      if reaction.message.author.id == bot_id:
        global reactions
        if f'{reaction}' in reactions:
          await VoteAdded(reaction, user)
          await reaction.message.remove_reaction(reaction, user)
        else:
          await reaction.message.remove_reaction(reaction, user)
  </code></pre>