[slashtags] update slashtags for dpy2

[phenom4n4n]

This PR updates the slashtags cog to work on discord.py version 2.0+. Currently, the cog can execute and add commands as expected. However, the following progress still needs to be made:

• Use discord.py internal API methods to add, process, and respond to commands
• Remove http models and use discord.py models for commands instead
• Integrate with red's slash command management system for restore command and automatically add commands without needing to be enabled