Add 'Explore topics' page

app/assets/javascripts/behaviors/shortcuts/keybindings.js

app/assets/javascripts/behaviors/shortcuts/shortcuts.js

app/controllers/dashboard/topics_controller.rb

app/controllers/explore/topics_controller.rb

app/finders/projects/topics_finder.rb

app/helpers/dashboard_helper.rb

app/helpers/explore_helper.rb

app/helpers/nav/top_nav_helper.rb

app/helpers/sorting_helper.rb

app/views/dashboard/_topics_head.html.haml

app/views/dashboard/topics/index.html.haml

app/views/explore/_head.html.haml

app/views/explore/topics/index.html.haml

app/views/shared/empty_states/_topics.html.haml

app/views/shared/topics/_dropdown.html.haml

app/views/shared/topics/_list.html.haml

app/views/shared/topics/_search_form.html.haml

app/views/shared/topics/_topic.html.haml

config/routes/dashboard.rb

config/routes/explore.rb