Refactor: User creation

[JuaniRodriguez]

Summary

When a student submitted an assignment and the user did not exist yet, the 'username' field was stored as 'NULL' in the database. This PR modifies the user creation process by adding the 'username' with 'email' value.

Evidence

[github-actions]

LCOV of commit d6db406 during CI workflow #60

Summary coverage rate:
  lines......: 95.6% (539 of 564 lines)
  functions..: 98.6% (72 of 73 functions)
  branches...: 86.0% (153 of 178 branches)

Files changed coverage rate:
                                                                           |Lines       |Functions  |Branches    
  Filename                                                                 |Rate     Num|Rate    Num|Rate     Num
  ===============================================================================================================
  src/commands/tarea/uploadTarea.ts                                        | 100%     23| 100%     2| 100%      6