Use `number_to_human_size` helper to show repository size

This will intelligently format large repository sizes in GBs (or,
shudder, TBs).

app/helpers/projects_helper.rb

@@ -200,7 +200,8 @@ module ProjectsHelper
   end
 
   def repository_size(project = @project)
-    "#{project.repository_size} MB"
+    size_in_bytes = project.repository_size * 1.megabyte
+    number_to_human_size(size_in_bytes, delimiter: ',', precision: 2)
   rescue
     # In order to prevent 500 error
     # when application cannot allocate memory