Simplify the code in courses/services.py so that creation of users is handled properly

[renatobellotti]

Text from #194 (just a more detailed description of and reason for the task):

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