Add case-insensitive unique constraint on User.username and email

[knolleary]

Closes #983

It took some digging, but I found a sequelize unit test for adding a unique index that uses the lower function. This means we can add a unique index to the table, without having to add another column:

indexes: [
   { name: 'user_username_lower_unique', fields: [sequelize.fn('lower', sequelize.col('username'))], unique: true }
]

I have tested and verified this constrain works as required on both sqlite and postgres.

Alongside this, I've updated the User.byUsername/email getter functions to do case insensitive queries and improved the feedback in the Admin Create User page and User Registration page.

[hardillb]

Looks like the errors from register/user api/v1/users post should be the same?

[hardillb]

I'm going to merge this. I'm 99.9% sure the tests are a GH artefact that this point in time.