Initial new_styles styleguide

fixes CNVS-14529

What it does:

- Adds feature flags for new styleguide styles
- Strips out un-needed variable highcontrast (these will need to
be set per element, not on the main color variables)
- Changes gemfile to point to temporary dress_code repo: we need this
so the dress_code generator strips out the parent tag

Test plan:

- get new gem: bundle install
- get new styles: npm run compile-sass
- regenerate styleguide: rake css:styleguide
- Make sure new_styles is disabled. You can do this in rails console:
Account.find(1).disable_feature!(:new_styles)
- Go to /styleguide and it should look the same as it does now
- Now turn on the feature flag. In rails console:
Account.find(1).enable_feature!(:new_styles)
- Refresh /styleguide and you should see are base starter styles for
the new look and feel!

Note: The new styleguide components are a wip and this request just
adds in the layout and styles specifically for the /styleguide page
to use while we work through all the components. You will notice
some funky looking ui - that's normal. We will be attacking these
components piece by piece.

Change-Id: I952b36346df77f98ddb7bbc5e27ab9b45ab4d8ca
Reviewed-on: https://gerrit.instructure.com/39600
Reviewed-by: Chris Hart <chart@instructure.com>
Reviewed-by: Derek DeVries <ddevries@instructure.com>
Product-Review: Colleen Palmer <colleen@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Matt Fairbourn <mfairbourn@instructure.com>