Adding checkin reminder setting quick action

What does this MR do and why?

This MR is part of the first iteration described in OKR checkin reminder settings per objective pro... (#422761 - closed) to add checkin reminder notifications to OKRs. This MR adds a per-objective setting for the reminder_cadence that can be configured via a quick action. The screenshot below shows an example of the quick action and some examples of work item notes.

Screenshots or screen recordings

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. In the rails console ensure that the okrs_mvc and okr_checkin_reminders feature flags are enabled:

Feature.enable(:okrs_mvc)
Feature.enable(:okr_checkin_reminders)

2. Create a new objective in a project by going to issues, and select the drop down next to New Issue and select New Objective. In the form input type an objective title and click Create objective.

3. Find the new item in the issue list and click on it. In the comment box type /checkin_reminder weekly and click the Comment button. A new entry should show up in the activity history to indicate the checkin reminder has now been set to weekly.

4. In the rails console, ensure the data is persisted correctly by fetching the record that was just created and inspecting the progress attribute.

work_item = WorkItem.last
work_item.progress.reminder_cadence # should return `weekly`

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.