Cleanup FOS user references

[kevinpapst]

Description

Some random cleanup commits with tiny BC break if someone relied on the fos_user_registration_register class or used the fos_user_content block (which I hope nobody did).

My main purpose was to get rid of the last fos_user references from the AdminLTE era - fixes #33

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• I updated the documentation (see here)
• I agree that this code will be published under the MIT license

[cavasinf]

Review requested but already merged 😄

Just to remember, there is also a PR on that #53
Modifying mostly those same files.

[kevinpapst]

Sorry, I really needed that at this time and forgot that it was weekend ... ;-)