Improve readability in some system specs

We're improving the readability of the ones we're about to modify.
Using human texts makes tests easier to read and guarantees we're
testing from the user's point of view. For instance, if we write
`fill_in banner_target_url`, the test will pass even if the field has no
label associated to it. However, `fill_in "Link"` makes sure there's a
field with an associated label.

spec/system/admin/dashboard/actions_spec.rb

@@ -47,8 +47,8 @@ describe "Admin dashboard actions", :admin do
     end
 
     scenario "Creates a new action" do
-      fill_in "dashboard_action_title", with: action.title
-      fill_in "dashboard_action_description", with: action.description
+      fill_in "Title", with: action.title
+      fill_in "Description", with: action.description
 
       click_button "Save"