Add Discord notification support

[lavgup]

Built this a while ago in lockdown when it was getting a bit difficult to track all the new resources coming in, then forgot about it. Little feedback that may be helpful for rewrites was that there was a lot of boilerplate in each notification service that just involved copy-pasting from other services.

Anyhow, thanks a lot for developing this program, have found it really useful!

[C0D3D3V]

Thank you so much for the great contribution :) I will merge it soon and add the missing functions (I wonder how your code can even work without implementing the abstract functions ^^ I have to test that myself).

Yes, the many boilerplate code can be found throughout the project, as far too little has been refactored and more and more has grown on moodle-dl without refactoring the code. However, I am grateful for any help to improve. Currently I'm in the process of converting all the requests and downloads to aiohttp and in the same course I refactor a little the code, at least small improvements.