Add shortcuts and counters to MRs and issues in navbar

app/assets/javascripts/header.js

app/assets/stylesheets/framework/header.scss

app/assets/stylesheets/pages/todos.scss

app/views/layouts/header/_default.html.haml

app/views/shared/icons/_mr_bold.svg

changelogs/unreleased/29866-navbar-counters.yml

features/steps/dashboard/todos.rb

spec/features/issues/todo_spec.rb

spec/features/todos/todos_spec.rb

spec/javascripts/header_spec.js