Change user & group landing page routing from /u/:name & /groups/:name to /:name

CHANGELOG

app/assets/javascripts/user_tabs.js.es6

app/assets/javascripts/users_select.js

app/views/projects/boards/components/_card.html.haml

app/views/users/show.html.haml

config/routes/group.rb

config/routes/user.rb

lib/constraints/group_url_constrainer.rb

lib/constraints/namespace_url_constrainer.rb

lib/constraints/user_url_constrainer.rb

spec/features/users_spec.rb

spec/helpers/projects_helper_spec.rb

spec/lib/constraints/namespace_url_constrainer_spec.rb

spec/routing/routing_spec.rb

spec/services/system_note_service_spec.rb