Username case insensitive lookup

[prawnsalad]

Currently all username lookups are case sensitive. We need to:

• Lowercase all usernames during lookups
• Lowercase all usernames on inserts
• Add a database migration that:
  1. For each duplicate username:
     1. Move all related networks user_id to the first user row
     2. Move all other table user_id columns to references to the first user row
     3. Delete all duplicate user rows other than the first

Simply using ilike or SQLites no casing options won't work as each option is different between each database that we support.