Handle decorators in class-parenthesis-modifying rules (#5034)
## Summary
A few of our rules look at the parentheses that follow a class
definition (e.g., `class Foo(object):`) and attempt to modify those
parentheses. Neither of those rules were behaving properly in the
presence of decorators, which were recently added to the statement
range.
## Test Plan
`cargo test` with a variety of new fixture tests.
