Formatter: `no_blank_line_before_class_docstring` preview style

[MichaReiser]

Implement the no_blank_lines_before_class_docstring as a preview style.

The new style aligns the docstring formatting between functions and classes.

class C:

    """The line above stays."""

Gets reformatted to

class C:
    """The line above stays."""

as it is already the case for functions.

Task: Copy over the test files, implement the new style behind a preview flag.