Add rubocop-rspec

In the spirit of https://gitlab.com/gitlab-org/gitlab-ce/issues/17406 and https://gitlab.com/gitlab-org/gitlab-ce/issues/17478, I found rubocop-rspec which has cops for a few things I'd like to automatically enforce, and I've got some other custom cop ideas we can add ourselves and/or send upstream.

This is its default configuration for 1.5.0, to which I've added inline notes:

# Probably not going to be able to enable this one easily
RSpec/AnyInstance:
  Description: 'Check that instances are not being stubbed globally'
  Enabled: true

RSpec/DescribeClass:
  Description: 'Check that the first argument to the top level describe is the tested class or module.'
  Enabled: true

# Hooray!
RSpec/DescribedClass:
  Description: 'Use `described_class` for tested class / module'
  Enabled: true

# We don't currently do this
RSpec/DescribeMethod:
  Description: 'Checks that the second argument to top level describe is the tested method name.'
  Enabled: true

# Nice to have but would likely require a lot of changes for not that much benefit
RSpec/ExampleWording:
  Description: 'Do not use should when describing your tests.'
  Enabled: true
  CustomTransform:
    be: is
    have: has
    not: does not
  IgnoredWords: []

# Probably good to have
RSpec/MultipleDescribes:
  Description: 'Checks for multiple top level describes.'
  Enabled: true

# Good to have but also requires a lot of changes for some of our older specs
RSpec/InstanceVariable:
  Description: 'Checks for the usage of instance variables.'
  Enabled: true

RSpec/FilePath:
  Description: 'Checks the file and folder naming of the spec file.'
  Enabled: true
  CustomTransform:
    RuboCop: rubocop
    RSpec: rspec

# Not sure, likely lots of offenses
RSpec/VerifiedDoubles:
  Description: 'Prefer using verifying doubles over normal doubles.'
  Enabled: true
  IgnoreSymbolicNames: false

# Hooray!
RSpec/NotToNot:
  Description: 'Enforces the usage of the same method on all negative message expectations.'
  EnforcedStyle: not_to
  SupportedStyles:
    - not_to
    - to_not
  Enabled: true

# Sensible
RSpec/Focus:
  Description: 'Checks if there are focused specs.'
  Enabled: true

# Likely never going to happen
RSpec/ExampleLength:
  Description: 'Checks for long example'
  Enabled: true
  Max: 5

I'll look into a custom cop for not supplying the default :each argument to before / after hooks, and would love to hear ideas for others.

cc @grzesiek @DouweM