UX: Add a link to "Add a new badge" on Badges page

Covered by behat tests.   For manual testing (if needed): Setup Log in as admin Access to "Site administration | Advanced features" and check that "Enable badges" is enabled. Create a course or use an existing one with, at least, a student and a teacher enrolled. Testing scenario 1: Administrator Log in as admin Go to the homepage and enable the navigation block (for boost) Access to "Site | Site badges" section CHECK the "Manage badges" and "Add a new badge" buttons are displayed   Testing scenario 2: Teacher Log in as teacher Go to the homepage and enable the navigation block (for boost) Access to "Site | Site badges" section CHECK the "Manage badges" and "Add a new badge" buttons are NOT displayed Access to a Course where this user will be enrolled as a teacher Access to "Badges" from the Navigation block CHECK the "Manage badges" and "Add a new badge" buttons are displayed   Testing scenario 2: Student Log in as student Go to the homepage and enable the navigation block (for boost) Access to "Site | Site badges" section CHECK the "Manage badges" and "Add a new badge" buttons are NOT displayed Access to a Course where this user will be enrolled as a student  Access to "Badges" from the Navigation block CHECK the "Manage badges" and "Add a new badge" buttons are NOT displayed