Discord 2.0.0 migraitons

Since discord.py 2.0.0 has been released for the support of the new interaction system, a few change has been one to the very core of the lib, including change that concerns bot using the "old way of doing commands".

Deciding to move to slash commands should be itself another issue, but the 2.0.0 migration for the bot to work on discord.py latest version is needed.

See: #37 