Separate the code for updating and creating user profiles

[renatobellotti]

There should be two functions:

• Update user profile (e. g. change address etc.)
• Create user profile (signup)

Especially: Remove the function get_or_create_user() in courses/services.py

Reasons:

• Creation of users should be done in only one place (DRY)
• No implicit creation of profiles means no multiaccounts
• Explicit is better than implicit
• a function should serve exactly one purpose: easier to read, test and maintain

[renatobellotti]

Also remember to delete the function get_or_create_userprofile() when you're done.

[renatobellotti]

Duplicate of #177