[BE] Issue analytics : Velocity MVC

Problem to solve

• Teams want to know velocity information for planning & predictability
• Closed vs resolved (#241383) is a pretty big change

This is the backend issue.

The frontend issue is #267194

Intended users

• Parker (Product Manager)
• Delaney (Development Team Lead)

User experience goal

Proposal

Now:

1. Add to issue analytics at group-level first
2. Add tab to issue analytics (Overview | Velocity)
3. Add area chart showing velocity over the last 10 iterations (or fewer if 10 not available)

Later:

• Add filter bar
• Persist filters while moving between tabs
• Duplicate functionality at project-level

Links / references