Store usernames in lowercase

[ItsOnlyBinary]

Hopefully i got all the places that are needed.

also unsure if i should remove the 'LIKE' from the where clauses as i think its now redundant

[prawnsalad]

Adding it here as it sets the username value will cover cases where a user is created via the model factory https://github.com/kiwiirc/kiwibnc/blob/34ad5bc1a786a62f679bf097469122194d94e311/src/libs/dataModels/user.js#L17

[prawnsalad]

As mentioned in #kiwiirc:

• Include a data integrity function that runs on startup that checks for any usernames containing uppercase letters (where lower(username) != username)
• If any are found, throw out an error explaining the issue and what to do before exiting.
• Include a config option skip_data_integrity_check, true/false. Defaults to false.