Update sfu module to use slash commands

[EarthenSky]

Commands in the wall_e/extensions/sfu.py module should be updated to use discord slash commands. @chanchantang has implemented the courses command using slash commands in https://github.com/CSSS/wall_e/pull/666, which can be used as reference for how a slash command is structured. Something like this might also be a good intro.

Since Jace has already rewritten most of the commands to use this new style, we'd like to be more consistent!

The commands to be updated are:

• def sfu
• def outline

bonus if you can rewrite the existing commands to be cleaner & more dry, I would be forever grateful!