Make profile setting able to update

[Blatzheim]

Description:
When going to /users/edit, users should be able to change a credential, even when they just supply information for only that credential.

For instance, when just click "Update" after not entering any information, an error is shown: "Current password cant be blank" - although it says under the password filed "Leave blank if you don't want to change it".

AC:

• Given that a user wants t change his email, when he only fills out the filed "Email" with a correct email address and click on "update", his email will be changed effectively (allthough all other fields are blank)
• Same as above goes for: Password (Password, password confirmation & current password as one)
• Same goes for the field "Lecturer Key" (As a student, i want to be able to upgrade my account to be a lecturer. #394)