Add !link command

[nicklambourne]

Adds a user accessible key value store, e.g:

This has the added benefit of adding the "brain" back into the bot and provides a template for further (and easy) use of the database though sqlalchemy orm models.

• [x] Still needs tests and more thorough manual testing.
• [ ] Need to set up the UQCSBOT_DB_URI env var in the server config.

[nicklambourne]

Looks good, thanks for cleaning it up. What should UQCSBOT_DB_URI be on the server?

I've added it to the config already. A lot of the pieces were already there, I may have had this idea before and forgotten to action it . 😅