Allow users to select the Files view as default project view

app/assets/stylesheets/pages/projects.scss

app/controllers/projects_controller.rb

app/helpers/preferences_helper.rb

app/models/user.rb

app/views/projects/_files.html.haml

app/views/projects/show.html.haml

app/views/projects/tree/_blob_item.html.haml

app/views/projects/tree/_tree_item.html.haml

spec/controllers/projects_controller_spec.rb