Add Readability category tag to articles

[soraia653]

This is more of a "Would be Nice to Have" feature, but I think it could really set us apart from other teams and at the same time provide something really meaningful for the teachers using our platform.

Tasks:

• Figure out a way to edit an article after that article has been created/updated.
• Apply grade calculation formula (Coleman–Liau index)

[soraia653]

The formula is pretty much ready!

[soraia653]