Show iterations for cadence

What does this MR do?

Add Iterations section to each cadence on the Cadences page.

Requires iteration_cadences feature flag

Also switches iteration form to use provide/inject for fullPath, to start making it use vue-router as well.

1. Add HAML and boilerplate - !59216 (merged)
2. Add Vue form - !59227 (merged)
3. Add vue router !60725 (merged)
4. Add basic List page - !61096 (merged)
5. Add collapsible cadence sections to list page [this MR] - !61546 (merged)
6. Add Edit page - !62587 (merged)
7. Delete cadence button - !62547 (merged)

Screenshots (strongly suggested)

Screenshot showing loading a cadence list with infinite scroll, and an empty one

Screen_Recording_2021-05-25_at_3.47.32_pm

Testing

To load the feature spec with extra iterations, apply this change and run test with

CHROME_HEADLESS=0 bin/rspec ee/spec/features/groups/iterations/user_creates_iteration_cadence_spec.rb

diff --git a/ee/spec/features/groups/iterations/user_creates_iteration_cadence_spec.rb b/ee/spec/features/groups/iterations/user_creates_iteration_cadence_spec.rb
index 2251f3ebcea..c57c10b3dd1 100644
--- a/ee/spec/features/groups/iterations/user_creates_iteration_cadence_spec.rb
+++ b/ee/spec/features/groups/iterations/user_creates_iteration_cadence_spec.rb
@@ -9,6 +9,7 @@
   let_it_be(:other_cadence) { create(:iterations_cadence, group: group) }
   let_it_be(:cadence_without_iterations) { create(:iterations_cadence, group: group) }
   let_it_be(:iteration_in_cadence) { create(:iteration, group: group, iterations_cadence: cadence) }
+  let_it_be(:iterations) { create_list(:iteration, 40, group: group, iterations_cadence: cadence) }
   let_it_be(:iteration_in_other_cadence) { create(:iteration, group: group, iterations_cadence: other_cadence) }
 
   before do
@@ -26,6 +27,8 @@
 
     click_button cadence.title
 
+    live_debug
+