Group iterations by cadence on issue filter bar

What does this MR do and why?

In the group and project issues page Vue refactor, behind feature flag vue_issues_list, the filtered search bar allows the user to filter by iteration.

This MR updates the list of iterations shown by the filtered search bar to be grouped by iteration cadences.

Screenshots or screen recordings

	Screen_Recording_2021-10-21_at_2.42.22_pm

How to set up and validate locally

1. In rails console enable the feature flags

   Feature.enable(:vue_issues_list)
   Feature.enable(:iteration_cadences)

2. Ensure a group has iterations e.g. https://gitlab.com/groups/h5bp/-/cadences
3. Visit the group or project issues list page e.g. http://127.0.0.1:3000/h5bp/html5-boilerplate/-/issues
4. Test the MR

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.