Cycle Analytics: first iteration

## What does this MR do?

- Implement the first iteration of the "Cycle Analytics" feature.

## What are the relevant issue numbers?

- Closes #21170 

## Screenshots

![cycle_analytics_screencast.gif](/uploads/d23c3c912caa6935fd47b53ca3a56b97/cycle_analytics.gif)

## Backend Tasks

- [x]  Implementation
    - [x]  Phases
        - [x]  Issue (Tracker)
        - [x]  Plan (Board)
        - [x]  Code (IDE)
        - [x]  Test (CI)
        - [x]  Review (MR)
        - [x]  Staging (CD)
        - [x]  Production (Total)
    - [x]  Make heuristics more modular
    - [x]  Scope to project
    - [x]  Date range (30 days, 90 days)
    - [x]  Access restriction
- [x]  Test
    - [x]  Find a better way to test these phases
    - [x]  Phases
        - [x]  Issue (Tracker)
        - [x]  Plan (Board)
        - [x]  Code (IDE)
        - [x]  Test (CI)
        - [x]  Review (MR)
        - [x]  Staging (CD)
        - [x]  Production (Total)
    - [x]  Test for "end case happens before start case"
    - [x]  Consolidate helper
- [x]  Miniboss review
- [x]  Performance testing with mock data
- [x]  Improve performance
    - [x]  Pre-calculate "merge requests closing issues
    - [x]  Pre-calculate everything else
- [x]  Test performance against 10k issues
- [x]  Test all pre-calculation code
    - [x]  Ci::Pipeline -> build start/finish
    - [x]  Ci::Pipeline#merge_requests
    - [x]  Issue -> record default metrics after save
    - [x]  MergeRequest -> record default metrics after save
    - [x]  Deployment -> Update "first_deployed_to_production_at" for MR metrics
    - [x]  Git Push -> Update "first commit mention" for issue metrics
    - [x]  Merge request create/update/refresh -> Update "merge requests closing issues"
- [x]  Remove `MergeRequestsClosingIssues` when necessary
- [x]  Changes to unblock Fatih
    - [x]  Add summary data
    - [x]  `stats` should be array
    - [x]  Let `stats` be `null` if all `stats` are null
- [x]  Indexes for "merge requests closing issues"
- [x]  Test summary data
- [x]  Scope everything to project
    - [x]  Find out why tests were passing
- [x]  Filter should include issues/MRs which have made it to production within the range
- [x]  Don't create duplicate `MergeRequestsClosingIssues`
- [x]  Fix tests
- [x]  MySQL median
- [x]  Assign to Douwe for review
- [x]  Fix conflicts
- [x]  Implement suggestions from Yorick's review
    - [x]  Test on PG
    - [x]  Test on MySQL
- [x]  Refactor
    - [x]  Cleanup
        - [x]  What happens if we have no data at all?
        - [x]  Extract common queries to methods / scopes
    - [x]  Remove unused queries
    - [x]  Downtime for foreign key migrations
    - [x]  Find a way around "if issue.metrics.present?" all over the place
    - [x]  Find a way around "if merge_request.metrics.present?" all over the place
    - [x]  Test migrations on a fresh database
        - [x]  MySQL
        - [x]  Pg
- [x]  Access issues
    - While the project is public and the visibility is set to "Everyone with access", you cannot visit the cycle analytics page when signed out.
- [x]  CHANGELOG
- [x]  Implement suggestions from Douwe's review
    - [x]  First set of comments
    - [x]  Second set of comments
    - [x]  Third set of comments
    - [x]  Fourth set of comments
- [x]  Make sure build is green
- [ ]  Make issue for "polish"
- [ ]  EE MR


See merge request !5986