Added User page

- asks user for a username if they're logged in and don't have a
username
- username is checked to be 4-30 characters, alphanumeric and unique
- added links to series and unit pages if the user that created it has a
username
- added user_name column to user table
Changed header to include link to current user
Attempted to merge from main
Fixed GitHub Conflict

Fixed two skilljoy.sql into one, ensuring that user table also has
username column
Merge pull request #17 from m-mmoocs/username-and-profile

Added User page